Not knowing the type of R/O unit, it is hard to tell. The newer R/O units purge water into the drain while it flushes the system, and that is what one of the barbed holes are for. I am assuming the other smaller fitting is for the outgoing water, and the threaded connection is to secure under the sink?
Your best bet is to Google your exact R/O unit and take a look at the pics to see if it looks compatible. I also know the big box stores carry replacement spigots, but again, not knowing your brand, I'd sit here all night guessing.
BTW, most new R/O units will run you about $200, but they carry 4-5 new filters. Have you priced your exact replacement filters? Hell, I just spent $120 for 2 of my 4 filters. It is damn near worth it just tossing the old unit and replacing with a new system. As with typical America, it is a throw away society anymore!