can -3x and 3x be like terms

No, -3x and 3x cannot be considered like terms.

Like terms have the same variable(s) raised to the same power(s), and in this case, -3x and 3x have the same variable (x) raised to the same power (1). However, they have different coefficients (-3 and 3). Like terms must have the same coefficient to be considered like terms.