I think about this often. With so many options in my daily recording life, the notion of imposed limitations is always appealing. Regarding synths, I did have a (very brief) moment where I'd moved my home studio from one room to another, bringing only the Prophet 5 and Moog Grandmother with me. Those two made for a great pair, sonically and aesthetically. I think I'd still wish for those two to be together. While I adore my Pro One, I use it almost exclusively for bass lines, and specifically in an 8th note synth pop stylee. And while I might be excited about limitations, any combo of synths I use should be about versatility and contrast. I can get the 8th note thing easily enough from the P5. I'd add the Korg Prologue as my 3rd synth. It can do lush, lovely, weird, murky and musical, yet with a distinctive tone I've adored since I first played this synth. It's got just enough MS vibe as well, for those moments I need the MS10 cat yowl happening. Anyway, yeah - the P5, Grandmother and Prologue would be my three synth picks.