Fellow book worms/authors, Do you have any advice for writing a character of the opposite sex?

Opinions from aspiring female writers preferred but opinions from aspiring male writers are welcome as well.

I have been brainstorming a fantasy novel for over 10 years. I feel like I am ready to begin a serious effort in my writing and intend to get this story published someday. The main characters are a trio of triplets (1 boy and 2 girls).

However, my writer's block comes from trying to write a convincing female main character. I have almost no problems with female side characters, but for some reason the main ones give me trouble. Any advice?
Fellow book worms/authors, Do you have any advice for writing a character of the opposite sex?
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