Hi, I have a fiat bravo 1.6 multijet diesel. The car is awesome!, clutch feels great 1st and 2nd gear, but when i'm on the highway just cruzing 3rd gear around 2k rpm's then just accelerate normal, it feels like the gear is slipping, then all sudden starts to grab and continue normal. It does not happen all the time.

Is the clutch slipping? or is my 3rd gear shot? She doesn't do it all the time and when I go up and down the rpm range, she pulls hard, no slipping. Seems to only happen sometimes and on 3rd gear only.

It sounds to me that the clutch is slipping.
Does the RPM goes up when you shift into higher gears before the speed increases?
You can try the clutch disk if it's slipping: start the enging, have the handbrake on, shift into gear and realease the clutch but not completly. If it's slipping, you would feel it, if not you will feel the engine pulling the car and will have the rear end of the car down to the floor due to handbrake. And you can do the test by shifting into all gears and see if for 3rd is anything different.

The engine does not rev up when changing to higher gears (2,3,4,5,6) and only rev up when change to lower gears on high speed. I was thinking about the slipping clutch as well, but the car wants to move forward when handbrake is on and in gears.

Would you say is a bad gear oil or fuel / fuel injector problem?

I would't say it's bad gear oil. Have you changed it recently?
Fuel/injector problem could be, but then again, the problem would manifest all the time, in any gear. But you could do a proper cleaning of the injectors, it will do them good.
I'd check the electrics: coil pack, spark plugs, leads.
Is there any error stored, is the check engine light on? Can you connect the diagnose to check the parameters?
Is the air filter dirty? Put a new one if it is dirty.

I didnt change the gear oil yet, but definitely will get it done soon. The car have about 60000km and it was changed some time ago. I will also get some injector cleaner, hopefully it works.

Thanks for helping out!