HI -- The day bed you mentioned takes up a lot of room.... it is going to be too big for the room.

Another idea you might look at is placing the bunks landscape style w a faux wall barrier. If your ceiling is high enough build it on a deck, so they can have storage underneath. For yourself, look for a sectional or sofa w pullout bed and storage below it. Something like this but with the wall running the length of the beds rather than the foot, would create wall space to place items against.

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Or a bed against the wall w/ a trundle for the girls. Would act as a sofa during the day. Trundle would be gone during the day but easy to pull out at night. Then the convertible chair in front of the floor to ceiling window across the room? (If I'm reading the floor plan right!)

I love the idea creating smaller spaces within a single space. In other words, you can have bunks for the girls in one area; a sofa bed in another corner and even some pillows and an armchair to bring it all in together. This way everyone can share the space and move around and the different spaces make the single space look larger.

Is this your full time residence? If so, do you rent or own? If you own, a murphy bed for yourself may be well worth the investment. A convertible chair or sofa is not suitable for regular use since you'll have to find a place for bedding, etc.

That's not an easy task. How high are your ceilings? Maybe you can built a kind of mezzamine in the kitchen/entry area, so you gain some extra space. Than you could have a convertable sectional as a second bed. Good luck!