Question

how to identify sn1, sn2 and sn3 reaction

Answer 1

The SN2 reaction is concerted. That is, the SN2 occurs in one step, and both the nucleophile and substrate are involved in the rate determining step. Therefore the rate is dependent on both the concentration of substrate and that of the nucleophile. The SN1 reaction proceeds stepwise.
There is nowhere the possibility that three reagents mix and form a compound.that is why sn3 not existing.

i hope it helps

Answer 2

sn1 reaction are nucleophilic substituents, involving a nucleophile replacing the leaving group (just like sn2) .however: sn1 are unimolecular: the rate of this reaction depends only on the concentration of one reactant. sn1 reactions happen in two steps:1.