My Bullmastiff is 3 years old and my Pit bull is 1 year old, from about three weeks ago they hae began to start figting, and sometimes very heavily. I don't know what or who is starting the fighting, becaue they were fine 3 weeks ago, does anyone have some advice or suggestions for me.

If they have been fighting for 3 weeks do you not watch them ?
If so you would be able to see who is starting the fighting and maybe why

I am afraid if your dogs have started fighting you are going to have to keep them apart permanently.
The pit bull is looking to dominate now, and the bullmastiff is holding on to pack leader at the moment
The pit bull will have more stamina as time goes by and will beat the bull mastiff in a fight as he gets older.
So unless you have facilities to keep them apart then serious damage even death will occur. The safest thing you can do now for both of them is to rehome one of them, In my opinion.
I wish I could be more helpful
Mari

I totally agree with Mari, keep them apart ALL THE TIME and try to rehome one.
Good Luck
Kelly