we changed the starter relay and it worked for about three trips then the next morning it wouldn't start

So when you hit the key nothing happens or it cranks and never fires up.

If it's just clicking, check for 12+vdc at the solenoid connection on the starter with the key turned to start. If it's there, and the connections on the starter and battery are tight and clean, change the starter. Tapping on the starter may free up a sticking solenoid plunger. AutoZone will test your starter for free. If it starts using jumper cables, you may need a new battery or at least a charge.

If you turn the key and the engine cranks but, won't start, check for spark at the spark plugs and fuel at the fuel rail.