Could be the tree isn't getting enough water; that would make the stems limber. Or it could be that's it's a naturally floppy variety like Aztec Gold.

You can always use green gardener's tape and tie the three branches together at a couple points or to a strong stake. I have a three-tip that was growing too wide so I just tied it in two spots and after the wood hardens the shape will stay in place.

I have one like that right now. I will be trimming it this spring when it is dormant. Trimming should help branching UNLESS it is a floppy one like Jen says, I.e. Aztec Gold.

I will be posting the same question, with a little twist. I can't tell from your picture, but my one plant with long branches, I notice that the branches have gotten thicker on the newer growth. I am curious if tis is because of my fertilizer. We will see what everyone thinks. Good luck. Jennifer