Unsure, looks right, only thing I can think of is to enclose the last two in brackets, so the division is done before the evaluation

=SWITCH

(

(Fields!Expr2.Value, "DataSet1") = 0, 0,

(Fields!Value.Value, "DataSet1") >= (Fields!Expr2.Value, "DataSet1"), 3,

((Fields!Value.Value, "DataSet1") / (Fields!Expr2.Value, "DataSet1")) >= 0.7, 2,

((Fields!Value.Value, "DataSet1") / (Fields!Expr2.Value, "DataSet1")) < 0.7, 1

)