Well, in the first place, i am not sure, if this is an official love triangle between the three or not.
I mean... he calls his girlfriend still girlfriend, but want to marry another woman... so that would mean he has two girlfriends for quite some time, because i am not planning to marry someone i have a very short relationship with.
If it is, i do not see the problem in it, because Engagement could be simple the highest level of their relationship that does not need to follow up with a marriage.
If it is not however.. well
or all three should try one
. It can be stressful sometimes and there will be times of jealousy, too, but it is also very exciting and rewarding.