Iranic in Slavic

Slavonic scholars seem to have a consensus concerning traces of a mighty
influence of an East Iranian source on the Common Slavic language. The
Common Slavic lexicon and a series of god names don't allow denying it.

Theoretically there could be 3 main sources of such an influence
(chronological borders can overlap, of course) -
1) Scythian (7th - 4th cent. BC)
2) Sarmatian (4th c.BC - 1st c.AD)
3) Alano-Yazygian (1st - 6th cent. AD - later it would be an influence on
single Slavic groups, not on Common Slavic)

However always when I met mentioning of the Iranian influence only the
Scythian variant was considered (usually 6th - 5th cent. BC were suggested
as the contact period).
Is there a _linguistic_ basis for such an approach?
Are we able to distinguish the Scythian loans from the Sarmatian and Alanian
ones?

Alexander