Is my account eligible for the new free-tier? I created mine long ago when a free credit of $10 is credited to every account.
Can I create 3 seperate 1GB volumes? Or only a 3GB is accepted?
Yes, the 3GB total in volumes is available to all accounts so you can create 3 separate 1GB volumes just fine.
Can I create three 256MB instances as well?
Or can I create one 512MB instance so that the overall usage stays below $10/month.
Yes, you can create 3 VMs of shared-cpu-1x with 256MB per month or you could create one VM with additional memory. We bill ~5$ per additional GB but since you’re using 512MB (256MB additional memory) it should be even lower than that.