Depends on the technological level
As your initial population is very small, in the first part (first millenium give or take a few hundred years), the demographics will be rapidly growing in the face of abundant land and food, lack of competition, and comparably advanced technology - animal husbandry and agriculture can provide abundant food while pastures and fresh farmland are available.
Then it will stabilize to the carrying capacity, and for the final 1500-2000 years the population will be mostly somewhere close to the capacity suited for technology of the time - the carrying capacity of the same plot of land grows as people get more effective with agriculture. 3000 years is long enough for major advances, but how far they will get mostly depends on your story.
You can simply look at real world analogues to where the population level converges at each technology level. Given the key data (58000 sq km peninsula with fishing), I'm going to take Denmark as an example (43000 sq km) and multiply by 1.33. [P.S. - maye I shouldn't have multiplied, as Denmark's arable land rate seems extraordinarily high].
Non permanent farming (pasturing animals, fishing, hunting/etc) tech level can support some 100k people in that area. 998
It will take many generations to reach that capacity (e.g. 1000 years assuming an average population doubling every 100 years), but they will inevitably get at least there unless they go extinct.
Iron age agriculture (which seems to be implied in the described tech that is "left to them") can get you to something like 500k - Denmark allegedly did, that may be exaggerated, but you're larger than Denmark. It must be noted that sea is a major "farm" in this case; pure farming requires more land for this population.
Medieval advances will allow you to get a million (again, as Denmark did) - mostly by more exploitation of nature and long term infrastructure development; replacing forests with farmland, irrigation, draining swamps, etc.
~1800s improvements seem to be highly reliant on introducing more effective species (e.g. potatoes) imported from Americas, so they might be or not be feasible in your world. Still, if your civilization is set at that level, then 2+ million would be reasonable.
Modern agriculture (tractors, chemical fertilizers) can feed 8+ million people in that area.
Futuristic agriculture (stacked hydroponics, etc) can go an order of magnitude higher, to 100 million maybe - if we're talking 3000 years from a technologically good starting point would be feasible, but is that what you want? 222