The PE side armor, in my opinion, is not worth the hassle of soldering (what it really takes to get it firm) nor the alternative of using super glue (which will break easily since you're talking about a complex Schurzen plates plus brackets).
It sounds like your first venture into PE. I would NOT recommend you get a PE schruzen set for a Panzer III. The nice thing about the PE schurzen is its thinness. But alternatively, you should just get some 0.010" thick styrene sheet and make your own. And you can affix it with glues you're already using.
Let me repeat: as a PE novice, I would NOT recommend you get a PE schruzen set for a Panzer III!!!!