it will be good if the ter 3 prototype cloaking device can be put on to any ship

but there will be an catch ... : the cloak will sometimes fail

Honestly, I'm struggling to figure out what you're talking about. The Oberth is a T1 sci ship, and it doesn't have a cloak. In fact, only two federation ships have a cloak, and they're both T5 ships, AND the cloak is limited to those two ships. Besides that, having a cloak on fed ships would be against the Algeron Treaty, and is completely non-canon.

http://en.memory-alpha.org/wiki/USS_Pegasus

Also, no. No phased cloak for Feds, please.

Too late, The Galaxy X and Defiant R already have those.

Already said and I can't find the button to delete this.

No ship have a PHASE cloak, please research a little on the matter, you will find that only the Pegasus had an experimental one, that later was used by the Enterprise D to exit the asteroid.
Also, the Pegasus class would require 4 nacelles.