I have been learning Mandarin for a little over 3 months.

wǒ xué pǔtōnghuà zì yì diǎr gèng yú sān ge yuè.

According to the English sentence above, I'd suggest that you put the pinyin sentence in this way,which would sound more identical to what you wrote in English:

wǒ xué pǔtōnghuà yǐjīng sān ge duō yuè le.

I have been learning Mandarin for a little over 3 months.

wǒ xué pǔtōnghuà yǐ jīng sān gè duō yuè le.