First you have to decide division. There is basically Open, Limited and Heavy. Limited is divided into magnified and unmagnified as is Heavy.
In the Limited classes, the only difference is the rifle sighting system. In magnified, the dominant optics are 1-6s ($1500 to $2000) and 1-4s ($300 to $2000). The Z6i and Burris MTAC 1-4 were the two most dominant scopes in 2012. I use the Burris MTAC 1-4.
Pistol...it is going to be a hi-cap 9mm most likely as there is no PF requirement. Even USPSA is now using a Time-Plus scoring system. It is going to be a Striker fired pistol (G34, M&P Pro, XDm, FNS for about $900 set-up) or a 2011 platform (figure about $2500 set-up). There are some CZs, but they are not prevalent. A few of the top 2011 guys have run .38 Super which is a little more reliable and takes spinners better (Iron Man). I shoot a USPSA Production legal M&P 9Pro with a very good trigger job and Taylor Freelance pads giving me 23+1. About $900 with mags.
Shotgun...it is going to be a 12 guage. Benelli is the most popular largely because they have been around for the longest in a configuration that will take the abuse. They are the lightest of the options, and kick the most. The bolts bounce out of battery and if you are fast, you can lock up the trigger. Just this year, they started to actively support 3 gun, maybe because they have been losing market share. Figure on $1000 on up.
The SLP is probably the second most popular, and what I shot in 2011 and 2012. It has the best stock trigger, soft recoil and is built like a tank. While it is ready to run out of the box, there are some improvements that can be made. Figure on $950 or so. There are various Wnchester SX models that have the same operating system and are a few hunderd less.
Remington 1100 has been a solid performer for many, but it is harder to load, the loading port can not be opened up sufficiently and when it starts to go bad, it goes very bad. Not an option IMHO, and I shot one for almost 20 years. Run around $600 to $900, might or might not need option, but a DMW loading gate is a necessity.
Remington now has the VersaMax. The number one 3 Gunner in the country is shooting it, so it can't be all bad. Good fit and finish, Benelli trigger and bolt with gas support if you will. Benefits of the Benelli and gas gun rolled into one. Expensive, but some feel worth it, $750 to $1200 depending on model. This is what I will be runing in 2013.
The Mossberg 930, in various forms has made some inroads, but it is not in the same league as the 3 above. Reliabilty goes away after about 6 or 7 thousand rounds and the amount of work on it to get it ready to roll is in the realm of $400 or so. Base guns in the $400 to $600 range. The JM models have hard chromed internals, so they last a little longer. Figure from $500 to $1000.
And then we have the basement dweller, sneaking up and smacking spaghetti around at times. The CZ 712 Utility. This has the same operation system as the SLP, however, not as good quality in the fit and finish. But for about $400 and the addition of a Nordic tube for $80, it is the clear bargain winner. $480 ready to roll.
A LOT of people say they are "3 Gunners" or going to shoot "3Gun", which is all great, but it has a large array of meanings. The majors typically require 200 or so rounds of shotgun in a match. Shoot 8 to 10 of those a year, with practice, the Mossberg 930 is a one year gun, the Remington 1100 maybe a 3 year gun. The SLPs and Benellis will last for many many seasons. OTOH, shoot a few locals yearly, 100 round of shot and less, keep them clean and the springs changed, you will get many years out of the Mossy and the 1100s as well. I shot 8 majors last year and shot just about 4600 rounds of shotgun total.
5 Years ago is was shoot a Benelli or maintain a Remmy. Now there are lots of good choices.
Rifle is going to be an AR-15. If you want the best set-up, it will be an 18" barrel with rifle length gas. If you are going to buy, your best bets are one of the "3G" versions. The Stag is very good, sans the comp. There are plenty of others from $800 to $3000 or so. If you build, skip the billet lowers, they cost more, add weight and offer no benefit. We could go on about barrels forever, but it is still going to end up being a SS 18" with rifle length gas. Adjustable gas is popular since so many ARs are overgassed. I took a divergent approach to shed the lack of reliability of adjustable gas. I designed a barrel with a smaller gas port and a gasblock that does not leak. Nordic ordered the barrels and they have been a huge hit in 2012. Lowers are about what you like, Seekins, Lancer, Firebird, Stag, JP are all quality and I like them all for different reasons. Triggers and comps and stocks are all very subjective with WAY too many fanboys. You have to shoot them to see what feels best to you. Most of us use JP Low mass carriers, or the Youngs Superlite. If you don't like those, go with a Mil-Spec.
My match rifle has a MagPul ACS stock with a 4 ounce carbine buffer and a Jard drop-in trigger in a Lancer lower. I run a Carbon Arms upper with the Nordic barrel, Carbon Arms gasblock, Carbon Arms carbon fiber handguard, Miculek comp and JP low mass carrier and bolt. The MTAC is in a JP one piece mount. Price, about $2000 with optic and it shoot 1/2 MOA and will run 1000 rounds with no cleaning 100%.
Then you will need a gear belt. An inner outer like CR Speed or AA, or a Safariland ELS. 3 to 4 pistol mag carriers, 2-3 rifle mag carriers and shotshell carriers. 2 years ago, it was row loading or a weak hand or strong hand load 4 (or 3) that frustrated many. About half of the active competitors are now using our TWinS loading as a primary or only loading system. You can figure on another $80 to $300 for shotshell holders if you go with twins or quads, or weakhand. Quality shotshell holders are about $10/shell. There are some cheap knock-offs, but we won't go there.