My opinion would be that as long as the window doesn’t raise the temperature above the rated range for the sub of 32°F to 104°F (0°C to 40°C) based on the product page, you are fine as far as audio goes. It’s possible that exposure to ultraviolet could cause the color to fade, and may become brittle with age, but I would not expect that until years of exposure.
I would likely look into getting some curtains if possible, or window tint. It’s a trade off if the sunlight is a big draw to the room or if the door is used frequently throughout the day. Maybe automated curtains?
edit: For some reason I was thinking we were talking about the sub mini, not Era 300. I don’t see the specs for Era300 off hand, but I imagine it’s the same or similar. And why do you have 3 Era 300s instead of a pair for atmos?
I'd add a speaker cover, bought are expensive but home made are a couple bucks. I have home made ones on my Subs.
Danny: thanks a mil for your response and suggestions, I have thought about a tint, but as the only viable solution. It’s a no-go from an aesthetic perspective. To answer your question, 2x Era 300s are connected in stereo and 1x Era 300 is paired with the sub.
Stanley: good suggestion, thank you. To avoid dust, I always cover all units with the inbox dust covers. Not sure if this could help with reducing the sunlight exposure temps
I'd not worry about temps, but if you are a good IR thermometer is about $20.
I use acoustically transparent cloth so there is no need to remove it.
Go with a lighter color to match your room for the least heat gain. Amazon will sell you 2 yards which should be plenty for the 300s, add a yard and do the Sub to match.