When to replace wheels

How long is a piece of string?

3,000 km of normal riding shouldn't have worn them out. If you race in crits, ride through winter, brake a lot or let the dirt accumulate? All these things will accelerate wear rates.

Do the rims have wear indicators? If you put a ruler across the rim is it concave?

If you buy some 3s I'd keep them for the better weather and continue to run your 7s the rest of the time.

Difficult to say because it depends on the type of riding you do and how much braking is involved. 3000k isn't much for a wheelset at all.
The Fulcrum 3 set is right in the sweet spot for value for money. I think they offer very good performance, markedly better than cheaper wheels but spend much more and you will suffer from the law of diminishing returns. Buy the R3's if you have the money and ride them on sunny days. Once the grit goes down in winter, switch back to the R7's. The R7's should last an age if you look after them and don't abuse them commuting or touring. When the sun comes out again next year, switch back to the R3's. Make life simple by buying a cassette for each set and putting GP4000S on the R3 and GP4Seasons on the R7's. Job done.