What type of Plecos were are they? How long have they been together? I am not sure on keeping Plecos with Bettas, someone else here will need to chime in. If it was indeed your Betta, he may not like having tank mates or they could have spooked him. Also, another reason maybe they were chasing him for some reason,so he defended himself. How is he acting with your other two? Sorry to hear you lost two.
Betta's: "Mint" & "Flare-Boy"
Other:6 misc catfish,Tetras,Glofish,Mollies
*RIP* To all my past little ones