We make a generic large reel that can hold 3,200m of F/O tether (4.2mm OD) and you might be able to fit 3,500m, but it would be tight. It already includes a single mode one pass optical slipring, as well as 6 electrical passes. You can see the photo below with 700m of 9mm tether spooled on it for reference. Retail price is $7,900 without the tether but you could make one based on this. The case is an SKB 3i-3021-18 for size and it’s about 60lbs empty.
We have flown about 800m down a tunnel and back with a Blue ROV on a single battery dragging a 4mm fiber tether. To get to 3,000m, you’ll likely need a bigger frame with 2 to 4 standard batteries. Just something to keep in mind.