Are they any seniors out there doing phd in biology? Im 3 months in and i feel like im making every possible mistake. Im trying to embrace the growth mindset and take it as a learning experience. But generally how long does it take to get used to lab techniques?

Six months to a year before you stop making basic mistakes maybe? After that, depending on the type of project you might spend the rest of your PhD mastering a few core techniques or you might be trying new things every month. If the latter, you can expect to always feel like you don't know what you're doing.

In general terms 3-6 months would be the norm but depends how often you're using them. If you keep getting something wrong ask for advice from someone else in the lab. It's okay to get help as you learn.