Hi Tony,
for what it is worth:
https://japan.observation.org/user/lifelist/40215
I spent something like 72 hours birding in Hokkaido between 13-15 of June 2014. It was a mad dash, starting in Kushiro after flight London-Tokyo-Kushiro.
We arrived in Kushiro in the rain in the evening of June 12.
We picked up a car from Toyota-rent-a-car (or was it Nissan?) around 9AM the next day (only available there during office hours), so june 13.
We drove towards Furen and picked up Red-crowned Crane along the road (this is a bit hit-or-miss but there should be cranes around Kushiro and maybe Furen Lake.
After checking in at Furen, we birded the forest South of Furen with Japanese Robin, Eurasian (Grey-bellied) Bullfinch (ssp. griseiventris), Sakhalin's Leaf-warbler, Bluetail and almost next to the lodge Middendorff’s and Sakhalin Grasshopper Warblers. In the small park east of the lodge we saw Black-faced bunting and Japanese Pygmy-woodpecker but not much else.
We tried to do some birdwachting towards cape Nosappu but there was not much to be seen there in the afternoon.
At night we tried to see Blakiston's Fish-owl in a small forested valley a bit West of Furen Lodge, and we succeeded. While waiting we saw many Latham's snipe (they are everywhere in June, above your head displaying, just like common snipe in the breeding areas in Europe).
The next morning (June 14) we tried cape Nosappu once again, very windy but the wind was a bit too much from the North-west so no close bird. The waves were the most spectacular I have ever seen though. Best birds were many Harlequin ducks, some gulls, both cormorants, Japanese Wagtail and Rhinoceros Auklet. I would have hoped for more Auklets but it was not the best day.
We still had a long way to go (heading to Rausu were be booked the Washi-no-yado) with not much birding on the way as it was quite rainy. Still plenty of White-tailed eagles (roadside bird!). I was accompagnied by a non-birding friend (we had a congres in Sapporo starting 16th of June), so we spent some time along the road stopping here and there and took time for lunch in Rausu. We didn's see much in Rausu harbor and on the road towards the pass, but we had a fine Pine Grosbeak up on the Shiretoko pass itself after a well-needed power nap in the car. We checked in early afternoon in the Washi-no-yado. We duly saw the Fish-owls perform half an hour after dark. Needles to say, we enjoyed our self-found bird from a day earlier much more!
Next morning (June 15th). We had our last day birding and had to be in Sapporo the same evening! So we got up very early, hoping for Grey Bunting and Brown bear. On the way up the pass, we saw Long-billed Plover but not much else and I searched for the bunting without any clue. On the other side of the pass where Bear sightings are most frequently, no sign of bear but still some nice flycatchers singing (Narcissus, Blue-and-white), an obliging Grey-headed Woodpecker, Short-tailed shearwater along the coast and a lovely little fox.
We didn't have any more time left so we drove across the island (with White-cheeked Starling and Bull-headed Shrike roadside), but not before trying to see some more forest birds near Akan Lake. Maybe not the best forest or stop, but the most convenient on our route. The best bird we saw here was a Scaly Thrush.
Unfortunately, there was no time to do any more birding in or near Sapporo with the congress going, social activities and a World cup going on (and lack of sleep!).
I hope this gives you an idea of what is possible in 3 days in Hokkaido. This itinerary would be significantly more efficient you can start and finish near Kushiro...
Biggest misses were Ural Owl near Furen (no stake outs with some info this should be possible), Siberian Blue Robin, Rubythroat (both robins no lifer though), Auklets and Red-faced Cormorant around Cape Nosappu, Grey Bunting and Japanese accentor at Shiretoko pass, Long-tailed Rosefinch along the road, Varied Tit , Japanese Thrush, Oriental Scops-owl in better forested areas,... Many of those birds could probably be seen without even searching hard, if you just have that little bit more time.