I grew from seeds two plumerias which are about 3 yrs; both are 4 ft tall and are sticks. When will they bloom? What do I need to do to get them to bloom?

The one on the left in the photo is blooming.

You should feed it with a high phosphate fertilizer. That's the middle number. I use something like a 10-55-10.

@TeamCII,
The one that is blooming is a old plumeria that has several branching.
The ones from seeds are growing straight up. OK, fertilizer...thanks.

Another question: to encourage branching, when do I cut them? Or wait till they are older?

You can cut whenever you like, it will just take longer for it to branch and bloom. I personally would allow it to bloom. Then it will branch on its own.

As @Dutchlady1 always reminds us plumeria are trees. Some plumeria trees grow tall trunks before they start blooming and branching on their own. Each variety is different. This year's fertilizing affects blooms for next year. Feed every 2 weeks with a good quality blooming fertilizer Excalibur, Nelson Plumeria Food. for example. Plumeria are HUNGRY trees!!