My birman cat gave birth to three kittens two days ago and she feeds them but sometimes she leaves them even though she hears them crying for her.
I try to force her to stay with them but I'm not around her all the time. What should I do?

She knows what she is doing. You have to leave them alone, and her.

Is she a first time mom? Tell us more if you want a good answer.

this breed is very gentle. Looks like a log haired Siamese, right ?

Just like human moms, momma cats need time away from their kids. As long she's feeding them and they're growing and healthy, I wouldn't worry. Kittens are deaf and blind for the first week or two after they're born. They're probably crying because they have no way of knowing where she is, and it scares them. Just keep an eye on them and make sure they're strong and growing.