Ok if you’ve gone to the effort and cost of a 3′ pad of concrete (did an 18″ slab for our summer house last year, although the thing did weigh around 600kg lol!) then I would get hold of some 4″x2″ (or wider) giving at least 2″ air gap, and lay them on the center’s of your floor panel or about 18″ so you end with an effective criss cross pattern. This will take out all the imperfections in the surface (there will be some) and make a solid as funk base.
Then floor down on that (screw em through if you really want/ exposed location) and rawl plug/hilti bolt into the slab if its really windy in that location), then sides up.
I always find with flat pack shed things if you just build them as they come, they are rarely square, rarely strong enough to last more than one winter (if any) and a nightmare to assemble. I pretty much always have to beef them up, even if its just some 1″ batton around each join screwed into both panels (wall to wall, and wall to floor, and wall to roof), it costs pennies and will really improve the thing.
As for security, if its having bikes in it, go crazy, really crazy. Make use of that massive concrete base and put a ground anchor into it, then floor on top of that so you can chain bikes to the thing. they could literally burn the shed down/knock it over, and bikes would still be chained to the slab. Ace.
