They say love is the strongest magic for a reason, and that doesn't mean spells and rituals. Love itself is a mystery and cannot truly be controlled or stopped. You're not the only one with influence, there are powers beyond you that may want that relationship to happen. If their love is true, no magick can taint it.
When you play with love magick, you're playing with fire. There never is a guarantee. If you need confidance in your abilities in magick, love magick is not the place to find it. The most experienced magickal practioners can fail when attempting to manipulate the hearts of others.
I suggest you do a reading (divination) on this love triangle to gain better clarity on the situation before choosing to go any further with it physically or magickally.