I live in FL. I have two children in Puerto Rico and one child in California, all are under 19 years of age. My question is what is the maximum amount of child support that can be deducted from my pay?

Puerto Rican (or Californian) law has nothing to do with it. You are working in Florida, right?

It depends on what the child support order requires as a monthly support amount, as well as upon what your monthly pay is. Florida (as far as I can determine) doesn't limit paycheck garnishment (although garnishment for other debts is limited by federal and state law to a percentage of the pay amount).

"15 U.S.C. § 1673

(a) Maximum allowable garnishment Except as provided in subsection (b) of this section and in section 1675 of this title, the maximum part of the aggregate disposable earnings of an individual for any workweek which is subjected to garnishment may not exceed (1) 25 per centum of his disposable earnings for that week, or (2) the amount by which his disposable earnings for that week exceed thirty times the Federal minimum hourly wage
...
(1) The restrictions of subsection (a) of this section do not apply in the case of (A) any order for the support of any person issued by a court of competent jurisdiction or in accordance with an administrative procedure, which is established by State law, which affords substantial due process, and which is subject to judicial review.
..."

As you can see, the federal restrictions on garnishments do not apply to child support.

The Florida agency which is responsible for collecting child support may, however, have rules which limit the amount they may collect from your paycheck. Check this site.

The limit is 50% of your gross pay. If it is over that then you need to go to court for a reduction of support.