I put up this one on amazon that seemed to work well. I sprayed it to be waterproofed. They sell multiple sizes, so there should be one large enough for 3 cats: https://www.amazon.com/dp/B09PH33KD1

I also put up this one, though once it got sprayed on, it smelled sooooo bad, so keep that in mind: https://www.amazon.com/dp/B01JCNF8X4?ref=ppx_pop_mob_ap_share