A former UFC fighter and his son were both shot during an armed robbery in Oklahoma Monday night outside of the gym where they were working out ... but both men are expected to survive. 

Mikey Burnett -- who last fought for the UFC in 1999 -- was leaving the gym around 7:30 PM when he was approached by two men ... at least one of whom was armed. 

Officials say Mikey's 18-year-old son saw the robbery and tried to fight off one of the robbers -- during the struggle, Mikey was shot multiple times in the abdomen. His son, Freddie, was shot in the leg. 

Cops say the suspects ran off and fled in a nearby vehicle.

No arrests have been made -- but officials say there is a ton of security footage and a bunch of witnesses. 

 Mikey has undergone surgery and is expected to recover. 

8:46 AM PT -- Cops have reportedly arrested 3 suspects connected to the robbery -- 2 men and a 15-year-old girl.