you aren’t putting a 3.5in in that slimline bay. you might be able to rig something to hold one there, though. that’s where the get ‘‘creative’’ part comes in.
you also have the 2x2.5in at the bottom but you’re missing the rails or caddies for them. should be like 5-6 bucks each or so on ebay or azn. don’t bother with those, though, unless you’re putting in hdd there. sata ssd can just be stuffed in the cage, they ain’t going anywhere.
be careful with whatever mb->sata power cables you have. make sure they’re the correct ones, as they aren’t all the same. you could get away with a 1->2 sata power splitter once on each mb power connection, as the output of each is designed for 2x3.5in hdd anyway.
edit: just looked at a similar model here. removing the slimline optical and stuffing a 3.5in hdd there will obstruct the cpu hsf a bit. might have to swap that for a lower profile cooler to put the drive there… BUT. if you take out the existing hdd (because its caddy thing gets in the way), there is just enough room along the flipout bit between that hdd bay and the 2.5in cage below to fit a 3.5in. would need a little ‘modification’ to the case there to rig a mount for it. but it fits. then the existing hdd can slide back in. (see pic, the ancient seagate hdd is placed in that space)
the number of years i’ve run usb->sata adapters and had (up to a dozen or so) bare drives laying around and propped up anywhere i could find a spot…