In general, 1 GB of ram is very low for todays hardware. Especiall when you take in account that a GB of ram costs just a few euros. You should increase ram to 2 or 4 GB on that server.
You should stop spamd daemon. Spamd is not used by ispconfig as the mail scanning is done with amavisd.
Is it normal that amavis is using 3 daemons?
Yes. You can set the number of daemons in the amavisd configuration file.