Hi Gera, welcome,
Unfortunately, you can't use CFS1 or CFS2 items in CFS3. The exception being sounds and sound files. You can use most engine sounds from the earlier combat sims and even some from the FS series, "usually" without any mods - just drop it in the main aircraft folder after removing the original sound folder. If you have let's say multiple P-51's, one of them could have a sound folder with all of the required sounds and the others would have a sound folder containing only a sound CFG file directing them to use the sounds from the first one - just like in FSX. This saves you from having to copy large sound folders into each aircraft folder. If you want to use gun or weapon sounds from the earlier combat sims then things get a little more complicated but not impossible.
Most missions include installation instructions, along with a list of required aircraft or vehicles needed. Ditto for most aircraft downloaded here at SOH. If you run into any problems with installation drop me a PM.