I think black out blinds or shutters might be a more practical and visually appealing approach.

Permanently cover the arch and blinds on all three windows would be level.

Shutters are good light blockers and are easily adjusted for different light needs during the day. I think they would suit the room and would be my suggestion, stained to match the ceiling.

Use three blackout shades you can raise and lower. There isn't room for bulky blackout curtains to be drawn completely off the windows.

A removable cover for the arch could be used in summer if you wanted a straight across row of blinds.

Have you checked that the windows meet safety requirements for egress?

Shades of some sort will provide the blackout feature.

with a ceiling rod. on each slope, hang curtain like a tent opening.

Pull back to ties when not needed.

From personal experience shutters are not necessarily good light blockers.

You need a visit from a local, a custom blackout cellular shade for the middle window and same on sides. Inside mount on all three.

EVEN with inside mount, there will be a minimal gap at the sides, as the head rail and shade material widths are never a match, can not be a match in width

Answer to that? A narrow piece of weather stripping behind the shade, on the casing.

A cheaper solution is blackout eye shades.: )

Is there another way? Yes

A trim panel board, fixed in place, over the arch portion to line up with casing at top of other two windows

Black out curtains, with a return to the wall to avoid light seep at the sides.

That arched window doesn't look like it's original to a historic house. When was the house built? If possible, replace the existing windows with 2 new double hungs. Then it will be easier to block out the sun.

And as applepieorder asked, it's very important that you comply with local laws when converting an attic to a bedroom. Where I live you need approved egress windows and a permanent fire escape.

That arched window is IMO no way original. As for window coverings I would do a fixed blackout blind in the arch and simple blackout blinds on the other 2 windows and those can open during the day but in S facing window probably kept closed will control the heat a bit better . The fact is in most cases egress windows a drop down ladder is a min. you need in an attic bedroom and where I had one we needed some kind of stairs on the exterior if no egress windw and ladder could be done . Attics are not living spce so be careful before doing this that all codes are met.

I kind of think Lyns idea could be the perfect answer since those drapes could be blackout for sure .