About the first one:

If we take this series from n=3 in absolute value, we can say it's less than:

All I need is to show that the series above converges...The problem is that the number of conjunct elements depends on n...

The second one:

The only problematic point is 0 ... We can substitute or something, but I don't think it's necessary ... Because we can say it's less than ... I can't figure out how to use now something like the comparison test... There must be an easy way...

About the third one- I've no idea

Thanks a lot!