Hi Ether, If the old crankset is a 130MM Bolt Circle, the smallest chainring it will fit is 38 Teeth. A compact (double) crankset wil give you lower gears and less torture to the chain than with a triple. I'm 50 and the old 42-52 cranksets always annoyed me. I like climbing like a caterpiller,not zig-zagging all over the road.
The cyclocross cranks are geared lower. You can get a trekking crank, seen on many hybrids (28-38-48) teeth pretty cheap. If you replace it with another double crank, the old front derailller should work. Triples have longer tails on their cages. Try Sheldonbrown.com and Parktool.com too. Chris