I had a very similar problem with my 4000T. Same issues. I swapped over the accelerator and that wasn't working. And then I noticed a dodgy ribbon connector. The thing fell apart in my hand. I replaced both the ribbon and the floppy drive and like magic the machine booted. I had changed the jumper setting to take the MKII and so flipped it back and refitted the PPC. The boot on the floppy had caused the issue and now the PPC was working and the whole thing sprang back to life. I know you have disconnected the floppy and hard drive but maybe you need one or tother to get the juices flowing. I sense this is a simple seating issue if you have the card connected and you may need some Brasso. Don't rule out a combination of issues though.
I hate towers by the way. It's a gravity thing. I have an A1200 in a tower and she works fine for a while and then she stops. Just like your 3000T. I swear that all I do is reconnect all the ribbons, press on the buffered interface, press the flicker fixer and reseat the Blizzard including the SIMM and she fires up first time no problem. Lasts a few months and then stops. I just press a few connectors and she springs back to life. Same with an Ateo Concepts 4000d in the tower. Seriously hate them. I've thought about lying them flat on their side... but they really are way too big.
I have two A3000T machines. Never use them. Just saying.
