The following are the steps to disable two of the previous three instances from automatic startup.
I issued the following command to check the status of these databases:
srvctl status database -d SID -v

I shutted down both databases with immediate option:
srvctl stop database -d SID -o immediate

Again I looked at the status:

Then I definetely disabled the automatic startup of those databases with the following command:
srvctl disable database -d SID

That's all.