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"I need three beds. Not 2 queens, or a king and a pull out queen sofa, but three beds."
How about two beds and a pull-out sofa? You might find something like that at an Embassy Suites or similar.
Another possibility is a room with two beds plus a roll-away bed for the second child. You'd have to check around to see if you can find a hotel that can put a roll-away in a room that already has two beds in it: might not be enough space in the room.
My guess is that a suites property with two beds and either a pull-out sofa or a roll-away in the living room would be your best bet.