I’m not too well versed in the miles game, but I have 2385 Delta miles, 6625 SAS points, and 7400 Virgin points. Individually they’re of no use to me, but I would like to pool them all together and put them towards a reward flight on Virgin Atlantic (and buy any additional miles I might need). Is it possible to transfer miles from one airline to another when they’re in the same global alliance?

No, it’s not possible. There is no point having memberships with programs from the same alliance in order to avoid the problem you face. Of course, I know SAS moved recently to ST.

In OP's case, not really. However, sometimes, if all three transfer to a 4th partner - for example Marriott Bonvoy, you can play games and transfer in and out of a hotel program, and then into each other. For example, if you have UA, SQ, and NH points, all of them (from what I remember) are partners of Marriott, and all in the same Alliance. You transfer points from programs 2 + 3 into Marriott, then transfer everything from Marriott into program #1.

Obviously, this creates horrible value, but at least it's better than seeing them expire, I guess!

That might be worth looking into. I’d rather get horrible value than having them expire. Generally, the only SkyTeam airline I use is SAS. My VS points are from my Tesco Clubcard, and my Delta points are from a recent domestic flight in the US (I don’t often fly domestically there), so whilst I tend to stick with one airline those Delta and Virgin points just sort of happened, and I may as well get rid of them. 🤷‍♀️

I have membership but only really use SAS. The other points happened to come my way and will not be added to in future, so I just want to use them now.

As others have said, transferring from one airline program to another is only possible in very limited circumstances, other than the ones mentioned above I am aware of:

1. If you have a Hilton account, you can transfer 10,000 VS miles into 15,000 HH points, which can then be transferred at a 10:1 loss ratio into certain airlines programs, of which Delta is one but SAS is not
2. Singapore Airlines and Virgin Australia have a unique program where you can transfer between programs, but at a 1:0.55 loss ratio each time
3. The IAG airlines using Avios allow you transfer between them, but that is limited to BA, Iberia, Aer Lingus and Qatar (Finnair uses Avios but I think does not allow points transfers)

If you fly mainly SAS, I would continue to credit there (including any DL flights), keep sending your Tesco points to VS, and be comfortable in the knowledge that DL points don't expire.

owjey, since you are a Hilton Honors (HH) Diamond member by virtue of over 30 stays last year, perhaps the best use of those VS miles for you would be transferring them to HH for use there.