Any time I change a weekly input on my iphone 11 or 13 calendar, it automatically changes it to GMT + 3, why? How can I fix this?

Hi MAP100,

Welcome to the Apple Community.

Seems the time zone in your location is 3 hours ahead of Greenwich Mean Time (GMT)

(Seems GMT helps us all with time-zone comparison workldwide.)

Apple support have articles online to help customise date, time, location etc. on iPhone

If you can’t change the time or time zone on your iPhone, iPad or iPod touch

Eg: You can:

Turn on Set Automatically

or

Turn off Set Automatically

https://support.apple.com/en-us/HT203483

See the time in cities worldwide on iPhone

https://support.apple.com/en-au/guide/iphone/iph1ac0b4af/ios

Change the date and time on iPhone

By default, the date and time, visible on the Lock Screen, are set automatically based on your location.

If you want to change them—for example, when you’re traveling—you can adjust them

Complete article: https://support.apple.com/en-au/guide/iphone/iphce20717a3/ios

OR

Change the language and region on iPhone:

You choose the language and region of your iPhone during setup.

If you travel or move, you can change the language or region

Complete article: https://support.apple.com/en-au/guide/iphone/iphce20717a3/ios

OR

Find settings on iPhone:

If needed, You can search for other iPhone settings you want to change

https://support.apple.com/en-au/guide/iphone/iph079e1fe9d/ios

Hope this helps you :-)