I had this same kind of problem with my Oliver 1755, and I took the injection pump gear off and eventually discovered the leak on the outside of the pump shaft tube. (I can't remember the official name of that tube)
I thought about doing this with the 990, hopefully if there is a leak in the front I will see it.
I sent the pump in when I rebuilt the tractor to get looked at since the tractor had not been running for a couple years. They rebuilt the pump for about $550. I had fuel getting in the oil and someone suggested it had to be the lift pump. I got a new one and that did not stop the fuel getting in the oil. So I was told to send my pump back in and have them replace the front seals. The guy told me they can last for 2 day or 10 years or whatever. So there is a chance that they are shot again. It seemed like the oil level went up pretty quickly after I had those seals replaced on the pump.
Anyway, I'm going to look more at the lift pump and if that seems OK I'm going to take the gear off the injection pump and see if any fuel is leaking out the front.
It has to be one or the other. And to leak 3 quarts in 25 or 30 hours of tractor use to me is a bad leak.