Hi. To get an HH:MM:SS format you can store your ingame timer in seconds and using following formulas to get minutes an hours:
M = (S / 60) % 60
H = (M / 60) % 24 - for 24 hours format or
H = (M / 60) % 12 for 12 hour format
Both / (division) and % (modulo) operators existing in BP.