Dennis is right. If you have 3 engines, I bet they are big. And bigger engines require FULL water pressure.
I do my F300's one at a time (5-10 minutes).
I'm also a believer that doing the flush when the engine is HOT makes a difference.
Accordingly, I'll leave an engine running while the one next to it is off and flushing. After that one is flushed, the HOT engine is turned off and immediately flushed. This is a tip a fellow Grady Owner shared with me.
Flushing while hot keeps your T-stats super clean. Hope this helps...