04 no intermitent no crank

[89 crew]

Hi, I'm having a problem with an 04 chevy 3500 6.0l. What it is doing, is occasionally it will not crank. the first time it wouldn't start I was at a store, when I went to start it no crank but I could hear a relay trip. after a bit of searching i found the three matching relays under the hood. I played musical relays. the truck fired right up I ordered new relays for those positions. everything worked great for three days and today it did it again. I haven't done much in the way of diagnostics yet, but I'm looking for ideas of where to start looking.