  In this work, quadratic reside codes over the ring F2 +uF2 +u^2F2 with u^3 =
u are considered. A duality and distance preserving Gray map from F2 + uF2 +
u^2F2 to (F_2)^3 is defined. By using quadratic double circulant, quadratic
bordered double circulant constructions and their extensions self- dual codes
of different lengths are obtained. As Gray images of these codes and their
extensions, a substantial number of new extremal self-dual binary codes are
found. More precisely, thirty two new extremal binary self-dual codes of length
68, 363 Type I codes of parameters [72; 36; 12], a Type II [72; 36; 12] code
and a Type II [96; 48; 16] code with new weight enumerators are obtained
through these constructions. The results are tabulated.
