A cardinal has been constantly flying into windows of house for 3 months. How do I stop without covering windows?

Net curtains? You can't really stop it without covering the windows. He either thinks that he can get inside, or he thinks there's another cardinal that he can see in the reflection of your window, and he's being territorial (or flirtatious?).

Maybe you could provide a distraction for him? A birdbath or a feeder or something?

He is probably attacking his reflection in the windows. I had a robin do this to all the windows at one end of my house one year. The glass was covered with smears and what appeared to be bird spit. I never thought of anything to do, but when breeding season was over, he stopped. The next year he did it less and then never again. You could try putting those silhouettes of predatory hawks or owls in your windows and see if that discourages him (and it probably is a him). Depending on the climate in your area, he may stop soon. Find out when cardinals stop breeding in your area and mark your calendar. I hope he stops soon! It sounds cute, but I know it gets kind of annoying after a while.

And it's sometimes downright dangerous, we have had pidgeons try it on 2 separate occasions, they went straight through the window. Blood, feathers and glass everywhere! Both times they managed to get out again. So hopefully they weren't too seriously injured.

...we have had pidgeons try it on 2 seperate occasions, they went straight through the window. Blood, feathers and glass everywhere! .

Holy cow! I had no idea they could do that.