Was thinking of getting a small 3-in-1 to learn on and eventually work on some guns and ideas for small parts . Currently I'm looking at a used Smithy AT300 for about $700-800. Any opinions on these machines or another type/brand?
My friend has one and really likes it. I've never used one (I did spend 25 years using professional metworking tools), but the Smithy should be fine if you understand what you get. There are always compromises in multi-function tools. It won't be the best lathe, nor the best mill, but for the purpose you describe, it should be adequate.
BTW: There's nothing quite as gratifying as looking at some fine machine work and saying to yourself "I made that!".
Just remember to disassemble, wash down and lube all moving shafts, gears and such before using it. They have a history of coming in with machining / casting grit in the guts, whether from HF, Grizzly or whomever.