3000fps was rarely possible with the 100/105gr bullets from a .243 Winchester.
Getting to 3,000fps with that weight generally needed specifically tailored max loads and barrels longer than 22".
Speeds around 2900fps at max was much more common with most rifles. But, either way, if you're not getting the desired penetration, no matter what the speed, you're not using the "right" bullet for the job. Cup & core, bonded, interlocked, partition, etc., all of them can do the job, if built "right" but choosing which ones are the best and which are adequate, and learning which are not is a learning curve that only actual performance in the field can teach.
A LOT depends on not just what you are using, but HOW you are using it, and how WELL you can use it. Around 40 some years ago, I met an old boy who had just taken his 6th elk with his .243. He considered the .243 an outstanding elk rifle! He shot Remington Core-Lokt and still had 12 cartridges left in the box!
He hunted deep timber elk, took only neck shots, always passed up any shot that wasn't what he felt was perfect. Six elk with six shots. Few of us would choose a .243 for elk, and fewer of us would or could use it as well as that fellow did. For what that fellow did, cup & core was quite successful.
Bullet construction matters, but what matters more is properly matching that with proper bullet placement.