Do I need PSUs with similar current rating when I connect them in series?
The maximum current that you're going to be able to draw for that configuration is lowest PSU rated current in the series connected supplies, which means 3A in your case. If your try to draw more, current protection/limitation will kick in.

I should confirm that the PSU outputs have to be isolated for the input, to be able to connected their outputs like that.

In reponse to your question: The PSU with the highest output voltage will source the highest current (if not all, it depends on the voltage difference and output stages) in a parallel connection configuration, not a series one.