I am 37 years old. What is the cause of inability to get another erection as quick as possible after first round of sex, even after staying 3 months without sex.

Sounds pretty normal.
What are you expecting?

Very normal if you did not have sex for 3 months, excitement issues.

Also at 37, it can take much longer for a second round than it used to

If you can even have a second round, you are better than many men

Simple, not waiting long enough.

Completely normal for an man to have a "rest period" of up to 45 minutes. Don't worry and don't rush it. Quality is much better than quantity.