It's Jacob's house they end up moving into eventually, because of the three of them, he's the only one with an actual house. Ezekiel moves from bolt-hole to bolt-hole, rarely stationary for very long, and Cassandra's flat is more like a particularly large walk-in closet. But being a world-renowned art geek can actually pay very well, and his maternal grandfather (the one that actually likes him) left him a bit of money, too.
It happens gradually, in slow degrees. Ezekiel's expensive cologne shows up in the bathroom. Some Cassandra's jumpers end up in the wash with his shirts. Books on astrophysics are on the shelf next to the Iliad, and there's a climbing harness and rigging hung in the hall closet next to winter jackets. Jacob doesn't mind; it's nice, makes it feel more...homey.
One night, Ezekiel and Cassandra come over for dinner like they usually do...and they just don't get around to leaving again. It feels good, to roll over in bed and curl against Cassandra's warm curves, or to wake up to Ezekiel sprawled over him like a climbing ivy. For all he likes to poke fun, the thief is a terminal snuggler. Jacob usually ends up in the middle, and that's just fine with him. He doesn't mind being the little spoon, just so long as he gets to hold someone else, too.
Other little things start to appear, making it less 'his' place and more 'theirs.' Like the proper copper kettle that has a permanent place on the stove, or the basket of knitting that's surreptitiously appeared next to the armchair in the living room. A lock pick set has taken up residence in the Odds and Ends drawer, and the new glass vase on the kitchen table is always full of flowers. There's never less than four different kinds of jam and jelly in the fridge, and there's Vegemite in the cupboard next to the peanut butter.
Jacob's the most domestic of the three of them. It's a role he's used to, since his old man crawled in his bottle and never came out after his mother died. Ezekiel says that he's their wife, and Cassandra laughs until she cries the first time she hears him say it. Jacob doesn't argue the term, though. One of them considers physics a hobby and not a career choice, one of them thinks that a heist of the British Museum is a romantic night out, and one of them can name every influencing style of architecture on a city block. Not to mention they work in a magical Library and have a carefully-rebuilt gargoyle as a pet. A stunning example of normality, they are not. If he's the wife, well...that's that.
A year after the 'official' moving in, Ezekiel gives him a frilly white apron like nobody's mother ever really had and a string of pearls. It's half done in teasing, half in sincerity. Jacob wears the apron when he cooks, though, and there's times when he'll lay in their bed and run the pearls through his fingers over and over. Ezekiel blushes when he does, and Cassandra asks one night, when they think Jacob's sleeping, if Ezekiel got them just as a joke, or if because pearls are Jacob's birthstone. Ezekiel doesn't answer, but that's an answer all it's own.
A few weeks later, though, and Cassandra finds a pair of silver earrings shaped like tiny owls with small diamonds set in their eyes, tucked away in her knitting basket. Ezekiel doesn't really wear jewelry, but when he finds a braided surfer bracelet with a piece of raw garnet tied into it, hooked on his climbing gear, he doesn't ever seem to take it off again.