Is it safe to use this relay with mains voltage?
I found a maybe non-official PDF: here
It shows the various ratings like 3V for yours, and a maximum of 120%, which is 3 * 1.2 = 3.6V, thus 3.3V is well within range.
It also shows (also on the relay itself) 0-250VAC so well within the 220-240V European wall outlet.


You can have 10A attached to it, use some safety margin, but it's quite a lot. In principle 220 (safe side) * 10A = 2200 W total devices. However, keep in mind some devices when switched on use a lot of current temporarily.

Btw, I also use these (not sure if I have the 3V or 5V version) without any problem with 220-240V, never had a problem. 

Always make sure the wires to the mains are NOT ABLE to be touchable, e.g. put them inside an enclosure, EVEN (especially) when testing.