After 3 yrs. my ex husband is now refusing to pay me my portion of his military pension.

It's worth a try to ask to resolve the problem without resort to mediation or court action.  Working it out between yourselves will save a lot of money, time and hassle.  If you reach an agreement, memorialize it with a signed document or at least an email acknowledging the agreement. The best way to do this is to contact the attorney that handled your divorce, he/she may be able to call or write the other attorney and work out the problem, then enter an agreed order if needed.   This  step is still cheaper than mediation or litigation generally speaking.

If all else fails, you have many different legal remedies.  You can obtain a judgment against your ex and then use various collection methods to seize his assets and wages to collect what you are owed.

If, in addition to the military pension payments, any child custody or alimony is not being paid, then you have such options as contempt of court (including having your ex-spouse put in jail for nonpayment), wage garnishment, and court-ordered seizure of certain assets -- even IRAs and pension benefits. You also can be awarded reimbursement of attorney's fees for having to chase after payment. When the amount owed is unpaid child support, you can also seek suspension of your ex-spouse's driver's license and have his or her tax refunds diverted to you.