Can multiple window blind motors run on the same low voltage wire?
If you wire it as you have shown, with three individual runs going up to your AC->DC converter, yes, probably.

18 gauge wire has a resistance of about 0.0062 ohms/foot (see this table and convert from ohms/meter).  So your 47 foot run will see about 0.6 ohms round trip; that works out to a voltage drop of about 1.2V, which is only 10% of your starting voltage.

Be sure to fuse everything properly and generally make the thing to code so that your insurance company won't have an excuse to deny your claim if a grease fire breaks out on the opposite end of the house.  What's "correct" would be a question for the diy stackexchange, and of course varies according to your local laws.