Javascript: 4 tokens (** based on broad interpretation of "assignment"!)##
Obviously my score of 4 is extremely generous/lenient!
To arrive at that score I've assumed "assignment" (worth 0 tokens in the question) includes such things as additive assignment, subtractive assignment, multiplicative assignment, and XOR-ing (^=) assignment
function f(a, b, c) {
d = a;
d -= b;
d = d >= 0;
a *= d; //a = a if (a>=b), else 0
d ^= true; //invert d
b *= d; //b = b if (b<a), else 0
a += b; //a is now max(a,b)
d = a;
d -= c;
d = d >= 0;
a *= d; //a = a if (a>=c), else 0
d ^= true; //invert d
c *= d; //c = c if (c<a), else 0
a += c; //a is now max(max(a,b),c)
return a;
}
If those assignments actually count the score is 14 :)