Kevin Garnett's younger sister is in serious trouble after cops say she tried to rob THREE check cashing stores in South Carolina ... while making bomb threats. 

It all started on Monday ... cops say Ashley Danyelle Phelps tried to rob a Check Into Cash in Greenville -- and told employees she had a bomb. Someone pressed a panic button and Phelps fled without any loot. 

The next day, cops say Phelps tried to hit another Check Into Cash store -- but failed so she went to an Advance America shop instead and once again told employees she had a bomb. 

Police responded and 37-year-old Phelps was captured a short time later.

She was formally charged with 2 counts of armed robbery, 2 counts of making a bomb threat and 1 count of attempted armed robbery.

She's currently being held in Greenville County Detention Center after a judge denied her bond.

A family source tells us KG and Ashley used to be close ... but they had a falling out and no longer speak.