The correct sentence is:
Yīnwèi xuéxí jǐn sān gè yuè, suǒyǐ wǒ de Hànyǔ hái bú tài hǎo.
因为学习仅三个月，所以我的汉语还不太好。
Because I have only been learning Chinese for three months, I am still not good at it.
In Mandarin Chinese, “因为 because…所以 so” should be used together to lead a reason and result clause. 
了 is a modal particle which is used at the end of a sentence to indicate that an action has finished. However, it can be used together with “仅(jǐn) only.” It is often used together with “已经(yǐjǐng) already.” For example:
Yīnwèi xuéxí yǐjīng sān gè yuè le, suǒ yǐ wǒ de Hàn yǔ shuō de búcuò.
因为学习已经三个月了，所以我的汉语说得不错。
Because I have already learned Chinese for three months, I can speak it well.