[Asus Transformer TF700] berndblb:
The first thing you should have done is upgrade your bootloader to the latest version. I would not actually use TWRP 3.0 for anything at this point if I were you. I have no idea if TWRP 3.0 works with that ancient JB 4.1 BL you got there, but I would not chance it. As a matter of fact, since you have to flash either stock rom or just the bootloader (and for both you use your recovery) I would highly recommend you flash an older TWRP version that is compatible with your BL. That combo - BL and recovery - is nothing you want to screw around with. You should go back to TWRP 2.4.1 or so.

TWRP Manager forced the latest TWRP version (developed for MM roms) on you and that's why I hate these apps! They do not check for compatibility at all....The proper way to flash a recovery is in fastboot and in fastboot only.

so I am going to give you a bunch of links and a general plan of attack, but you will have to do a fair share of reading.

First things is: Get fastboot working on your PC!!!!!! The first part of this guide I wrote way back tells you how: http://xdaforums.com/showthread.php?t=2688891

Once you got that working flash TWRP 2.4.1 - instructions on how to are in the same thread. Reboot the tablet, then boot into TWRP.

Now flash the latest bootloader (Packaged with a slightly younger TWRP version from here: http://xdaforums.com/transformer-tf700/general/firmware-10-6-14-10-twrp-t2867157

Reboot the tablet, then boot back into TWRP.

Now you again need to update your recovery to a version you can use with a current rom. I'd recommend this baby and it comes in a zip you can flash in your current recovery: http://xdaforums.com/transformer-tf700/development/recovery-kang-twrp-tf700t-2-8-x-xarea51-t3049395

Flash the Kang recovery, then reboot and boot back into TWRP.

Format your data partition in TWRP. It takes an hour or so but it does a great job of leaving you with a clean slate for the new rom and is required before you flash a major change (which you are doing in any case).

Now you are ready to flash a rom.

I am going to recommend a really old one and a really new one. There are plenty of others to choose from in the Development section, but none of them is still under active development.

CROMi-X Resurrected. Based on Asus stock anno JB 4.2.1 but modded and fine tuned to the gills. LMK still puts out an update now and then, but those are more fine tunings of the fine tuning. Very slight changes - nothing major is going to happen here anymore. But it's a very stable and well working rom for those that actually like the Asus interface. http://xdaforums.com/transformer-tf700/development/cromi-x-5-4-resurrected-tweaks-t3273515

KatshMallow by Timudru. Yeah, Android MarshMallow for the TF700. Most Android users are still dreaming about MM and you can rock it on this ancient machine. And it's a beauty, just absolutely unbelievable what Tim has cooked up for this hardware. Trust me! Fantastic rom! http://xdaforums.com/transformer-tf700/development/rom-t3282166

For both of them you should convert your data partition to f2fs which you can do in recovery. Instructions are here: http://xdaforums.com/transformer-tf700/general/guide-convert-data-to-f2fs-twrp-2-8-t3073471

Actually, for CROMI-X Resurrected you don't have to do this - it's build into the rom installer.

You could try to short cut all this and just flash a new bootloader/recovery combo in your current TWRP 3.0. Maybe it works. Maybe it hardbricks your tablet. Nothing worse than trying to do something substantial (like flashing a bootloader) in an incompatible recovery....

Ok, now I shut up and let you read.

Have fun!