Innodb_Buffer_pool_size in a vps with Pesl Onyx ad more than one Mysql installation

Hello . Greetings to all group members. I have a confusion about mysql db configuration , can anyone help me please ? I have a VPS server with 50 GB ram garantee. On this server we installed Plesk Onyx. Than open 3 account for 3 customers. Now I have to setup .cnf file. Talking about innodb_buffer_pool_size 80% of ram is the raccomanded size. But haviving 3 database instances under plesk and 3 .cnf file… I have to be sure that the sum of every innodb_buffer_pool_size do not exceed 80% ? I mean DB1 40% DB2 20% DB3 20 % … or what other.?? Thanks a lot for every answer.