Hi

If 43 databases were single instance database that would have been okay .But you still have the problem of sharing of resources.

If out of 3 you have 40 RAC instances then you are in a big soup mate.
RAC was designed to elimnate a single point of failure and dsitribute workload your design works exactly against this a single bad program/query would bring down the entire node because of RAC now .

I sympathise with you as your option is very attractive from a management perspective but technically useless.

Better option would be to use VM and virtualize the environment and have one database per guest OS and then cluster with another host sharing resources in RAC is a very bad idea without a VM.