This is one of the things that never did get properly errata'd for 5e. Per the rules for Area Indirect Combat Spells (pg 283, SR5)
Area indirect spells
travel from the magician to the point of detonation and
then go boom. The test is like that for grenades (p. 181):
a Spellcasting + Magic [Force] (3) Test with scatter of 2D6
meters.
This is relevant to your example, as Blast (pg. 284 SR5) is typed as an Area Indirect spell. Ergo, Rikki rolls against a Threshold of 3 and Gangers 1 and 2 roll nothing, just as if Rikki were instead throwing a grenade. The exception to the grenade rules, as said in the following sentence after the quote, is that Rikki's net hits adds to the DV.
Of course this is problematic for counterspelling, and indeed directly contradicts the example for countespelling a fireball (on pg. 295 SR5). Furthermore, pg 190 mentions a -2 dice modifier for defending against blasts from grenades and spells. But, per the rules for blasts (pgs 181-183 SR5) there is no defense granted against such attacks in the first place, so the citation in pg 190 is flawed to begin with.
If I get to participate in a 5e CRB errata project, this is literally the #1 item on my list to address. In the meantime? My personal opinion is that grenades and area indirect combat spells grant no defense test. Counterspelling, if used against an indirect area combat spell, subtracts from the caster's net hits rather than adding to the defenders' non-existent defense tests. Arguably, a result of 0 hits on the casting test results in no blast being generated at all, rather than one that deviates with a margin of failure of 3.