As i know that PS3 doesnt support screenshots on GTA V. but I found a way to make a screenshots and download them in your phone and post it on Instagram
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Step 1: RSG Social Club Account
Start up GTA 5, and pull out the phone of your favourite character and make a photo from anywhere you like
Then upload it to the gallery. and your photo will be on your account
and then Sign in on your Rockstar Games Social Club account or use the IFruit app
Step 2: Download the Picture From Browser
In the IFruit app. tap on Snapmatic
Look for your picture and tap and hold the photo and tap on "Save picture as"
(If the name is 0_0 and the file already exists, just rename it to what you like)
Your photo is downloaded and ready to be uploaded
Step 3: Extra's for the Photo
In GTA 5, you can add filters and change the frame, and you can make selfies from certain events, missions and crazy people you encounter in game
Step 4: Gallery
Here are examples from my game experiences, from high to low
What do you think of them?