honestly there are more lol...if ur looking for 3 to mount on a-pillar i would say boost, afr, egt
you can mount a fuel pressure gauge like in the glove box, it is not necessary to monitor it at ALL times, i like to think of it more of a diagnostic gauge...it is more important to have afr hands down...egt is helpful but if i had to choose only 3 it would be boost, and 2 widebands but thats just me