I can______you with your math -------help
can+动词原形 can 是情态动词,should,must,would,could等均是此类用法。
She often_____me with my math ------helps
she 第三人称单数,动词加s
often: 经常发生的,惯常动作。 所以用一般现在时。
She_____me with my math.----------helped
鉴于你的三个选项,排除后,就选helped,表示过去发生的事,用一般过去时。但是它也可以填 helps。
I can__help____you with your math.
She often_helps____me with my math.
She__helped___me with my math.