If i start doing crossfit for 3 days then 1 day of rest, do i need to still go to the gym and lift weights?

Depends on goals: Crossfit has muscle resistance training component. For overall fitness, a 3 day on, 1 day rest cycle is good. Remember, a workout causes certain stress to the body, so appropriate recovery time is also very important. If you desire certain muscular gains, then you may want additional weight training. For many, crossfit and appropriate diet can result in signficant body development.

You can be fine: As many Crossfitters only participate in Crossfit work out without any supplementary gym use. It really depends on your fitness goals, if you feel like you'd like to add supplementary resistance training to a Crossfit regimen.