There is nothing that says 50/50 can't be sub-ohmed.
It is a bit thinner; so flooding/leaking
might become a bit more problematic in
certain atomizers. And, as
@bombastinator already pointed out, there
could be excessive
throat-hit, and/or irritation (for those with PG intolerance) for some individuals. But, otherwise, there should be no prohibition.
I would first try
@JCinFLA's suggestion, and contact the vendor to see if they would be willing to correct your order. If they are willing... problem solved.
If they can't/won't... then you have a couple of other choices. Either,
vape it as is. Or, order an equal volume of 3mg/mL nic in 100% VG, and blend the two bottles together. The result would be a 25PG/75VG/3mg/mL mix. That should be close enough to your usual blend, that you shouldn't(?) notice any difference. You'll just have twice as much.
