Hi! Recently I bought Renek just for 3s, but I don't know how to build him. I would appreciate if someone could help me with the build, runes, and some tips. Thank you!

well, I'm not a Renekton pro, but have it on good authority that building him should usually go as follows:

Masteries are up to you, 21/9 or 9/21 depending on how aggressive you want to go early on.

Runes are apparently best as ArPen Reds, Flat Armor Yellows, MR Blues (either flat or per level, your choice) and Flat AD Quints.

For items, you always build according to the enemy team. Facing 3 AD champs? Rush a Sunfire + Ninja Tabi. Facing 2 AP with lots of CC? An Early Hexdrinker + Merc Treads seems to do the trick.
General item options for him beyond what I've already mentioned are: S Visage, Randuin's, Last Whisper, Brutalizer, and sometimes Blade of the Ruined King can pay dividends if you hit someone with a 50-fury Stun.

Thank you! I will be trying that

Any other suggestions, tips or anything are welcome

I like to go with Brutalizer and Mallet as initial items and then go tanky. Aegis is good on just about anyone and Renek is no exception. Randuin's and Sunfire, or turn the mallet into Frozen Mallet if we can afford to be more aggressive. Basically Renek doesn't need much to do damage, and the longer he stays in the fight the more disruptive he can be. That's why I prefer 9/21/0 masteries. Build to survive.

Nice! Thanks for the tips, I have been building like that and I'm winning every match. Thanks!