Can you post a Photo?

Are the three main stems you referred to planted very close together? Is there a possibility of you trying to separate them so you will have three trees instead of one?

If separating is not an option maybe you can train by pruning it like triple trunk style.

A quick result is just remove the unwanted growth to expose the full uniqueness of naturalistic stem structure. Oleander naturally sprouts readily and is quite forgiving when pruning so enjoy your pruning journey! Oops, maybe I should keep quiet.

{{gwi:7760}}