formula

$&&P(M_{t_1}^{(3)}\in A_{t_1},\ldots,M_{t_n}^{(3)}\in A_{t_n})\\ &=&\frac{1}{2}P(W_{t_1}^{(3)}\in A_{t_1},\ldots,W_{t_n}^{(3)}\in A_{t_n})+\frac{1}{2}P((-W_{t_1}^{(3)})\in A_{t_1},\ldots,(-W_{t_n}^{(3)})\in A_{t_n})\\ &=&P(W_{t_1}^{(3)}\in A_{t_1},\ldots,W_{t_n}^{(3)}\in A_{t_n})$