Can I make a sub from two three-ways?

Probably not. Few so-called full range speakers have adequate low frequency extension, linearity and power to function as a sub. You may get some more bass and like it but, at the very least, you will need to add/build/buy appropriate crossovers.

Self powered, pre-engineered for easy interface subwoofers are too abundant and too cheap to pass up, UNLESS, you want to do the MAJOR DIY. With a ready made sub, you can always sell it and get most or some of your money back. Even 15 inchers that are ported are pretty fast and tuneful. I would advise that first for 6 months, and then see if you really feel you are missing something.

OK, thanks, just a thought. I'll get a sub.

rev