How can I make a simple "pirate's costume" for a 3-yr-old boy?

Most of it could probably made simply by assembling the right outfit... a black and white striped shirt with black pants and boots would be a great start. You could make an eye patch out of felt and elastic, and then you could buy a pirate hat and a toy sword.

Black sweats tucked into boots, white collared shirt tucked in, red sash tied around the waist (us a scarf) eye patch and do rag on the head. Very easy! With makeup draw on some tatoos or scars on the face. May a fake saber and rubber knife stuck in the waist band. Very easy!

Red scarf over the hair. You can get a pair of clip on hoop earrings and use one of them. Black pants that you cut at the bottom so they look worn and jagged. Oversized boots. Add a vest. If you can find a hat, eye patch & sword that would top it off.

I made a pirate costume a long time ago for my son when he was about the same age. I used a white turtle neck shirt. Cut the sleeves to look jagged. Then I used a permanent marker to draw a scull and cross bones on the shirt. Pants can be cut jagged too. Use eye liner to draw scars. We had a stuffed toy bird that I sewed on the shoulder. He made his own sword from lego blocks. A pumpkin bucket for candy and he was all set. You can get most of this at Good Will, if you don't already have it. Best wishes and Happy Halloween.

Lots of photos here. Click on a photo and go to the site.

https://www.google.com/search?q=diy+pirate+costume+for+boy&tbm=isch&tbo=u&source=univ&sa=X&ved=2ahUKEwjL-aD18PDdAhVMyoMKHdvlDFgQsAR6BAgDEAE&biw=1524&bih=734&dpr=1.25