  In a recent paper A. Merkurjev constructed an exact sequence which includes
as one of the terms the group of degree 3 normalized cohomological invariants
of a semisimple algebraic group G, greatly extending results of M. Rost for
simply connected quasi-simple groups. Furthermore, in the aforementioned paper,
Merkurjev uses his exact sequence to determine the groups of invariants for all
semisimple adjoint groups of inner type. The goal of this paper is to use
Merkurjev's sequence to compute the group of invariants for the remaining split
cases, namely groups of types A and D that are neither simply connected nor
adjoint. This description not only demonstrates the existence of many
previously unstudied invariants but also allows us to extend several known
results which relate these invariants to the Rost invariant and algebras with
involution.
