How common is it to reclassify to 11B after 3 years as an 13B?

The easiest way to reclass to 11B is to go to Ranger School

Edit to the above:

“If 11B is understrength or balanced”. 11B is hardly ever understrength, but usually balanced. It depends mostly on if 13B is under, or balanced.

Yes, it is possible. When you get to your 2 year mark, your reenlistment window will open up (15 months prior to your ETS date). When that happens your retention/reenlistment NCO will be contacting you to decide if you want to re-up or finish out your full 3 years and get out. At that time you can request reclassification. Keep in mind this will only be possible if 11B is “understrength” at your rank and 13B is “balanced or overstrength” at your rank. The Army is changing so in 3 years it may be more difficult. But anything is possible. Just make sure you make the reclass as soon as you can in your career because the more time in service and rank you make, the harder it will be to reclass.

Lastly, if they do not let you reclass at the end of your term, you can always go to Ranger school and then put in a 4187 to switch over to 11B, since it will become your secondary MOS, given that you successfully graduate. Either way, good luck to you. Make it happen.