Ex-MMA star Jason "Mayhem" Miller has pled guilty in his domestic violence case -- after allegedly roughing up 3 different girlfriends between 2015 and 2017, TMZ Sports has learned. 

Court docs show Mayhem was charged with 5 counts of felony domestic violence in Sept. -- along with 1 felony count of vandalizing a GF's car and house. He was also hit with misdemeanors for damaging 2 girlfriends' cell phones to prevent them from calling cops.

Miller initially pled not guilty to all charges -- but later struck a plea deal with prosecutors in which he pled guilty to corporal injury to a spouse/cohabitant and the rest of the charges were thrown out. 

Miller was sentenced to 3 years probation with a suspended 4-year state prison sentence -- which means if he violates his probation, the judge can lock him up for 4 years. He also got credit for the 62 days he already served in jail. 

Per the terms of his deal, Mayhem will be released to a sobriety treatment program and must remain clean for the entire term of his probation.