I agree with @Otherpeter
“Bad” is all relative. A “bad” hero might be good if it fills a gap in your roster. My roster is very lacking in 4*/5* green so I still use many 3* green (including Carver and Isshtak) for my war hits sometimes if we’re facing blue tanks. And you don’t need any rare mats for 3*s so really they’ll give you more in return than you put in especially if you’re F2P
Totally shameless plug now but if you want to see them in action XD 
(This was a good board and a bad defense team set up haha I know but they can be pretty good):
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