[transmetahuman]

1) You don't "need" any of the Tech books to play in any setting. They're just very useful. You can just wing it, using the lists in Basic as a beginning frame of reference. Google/Wikipedia the real world objects when something comes up you didn't foresee. Future tech you need to give some thought to, but you can just pick a favorite sci-fi book or series and go by what they have and don't have.

There are hundreds of RPGs in different settings that have a tiny fraction of GURPS's equipment info (if any), and it's never stopped anyone from playing them.

2) Even if you have all three books, I guarantee that at some point you'll want info that none of them have. You'll have to do one of those things up there anyway. One book, two books, or all three, the only thing that changes is how often you have to wing it.