I carry my Brompton quite a bit, ~1/4 mile per day while touring (most of it to/from wild camping spots). With all the accessories, tools, lock, water, etc, my MR6 weighs ~34lbs, plus another ~15-20lbs (of camping gear, food, water) in the T-bag carried in a backpack mode. Once even hiked the whole set-up up a 30-story staircase.... and I’m a 155lb retiree
.
My advice for a 3 story walk-up is to learn/practice how to carry the bike
on top of your shoulder. For example, if you request delivery of a 5gal water cooler jug (~40lbs) to a 3rd floor apt., I can virtually guarantee the delivery guy will becarrying that jug on top of his shoulder.... and
never carry it like a suitcase down by his leg - you know, like how most folks carry Bromptons.
For even longer distances you can also carry the bike, folded or unfolded, across both shoulders - like carrying 2 water buckets with a pole. The Brompton is unique to these carry methods being so compact and with a tight locking fold. Just watch your ceiling/doorway clearance if you are a tall person.
Just my $0.02, YMMV and all.