Tri-axle dump truck mudflap issue with mudflap in front of lift axle

You can buy a gard it is sold at most truck stop and it should hold it in place

 

Replace the mudflap with a 1/4 fender. Or remove it altogether. It isn't required to be on the truck, only keeps mud & road grime from being tossed forward towards the cab.

 

Most common style. Or hang a loop of chain from the two outside bolts , let it hang almost to the bottom of the flap.

 

They are called "anti sail" guards... Now, while a flap is required at the rear, one may not be required at the front. Nonetheless, the are are called Anti-Sail brackets, and they aren't very expensive!

 

Sometimes guys will run short mudflaps there instead. If the lightweight anti sail bracket doesn't work guys will run a sustantial aluminum upside down T bracket on the rear of the flap.

 

If you're hauling out of a quarry or rock pit you can usually find some old conveyor belt. Cut to fit, drill holes for the mounting bolts, and you're good to go.
That stuff lasts forever.