Hello, Julie! Thanks for your question!
I could live at the Disneyland Resort, so I certainly understand anticipating the need to add extra days to your visit! You can absolutely upgrade your
ticket to add extra days, up to five days, by visiting a ticket booth at the Disneyland Resort. You will only pay the difference between a three-day ticket and four-day ticket. However, you must make this upgrade while your three-day ticket is still valid (so before the end of that third day) and the extra days you upgrade to must be used within the original 13-day expiration window that applied to the first ticket. You can also upgrade your ticket to a Park Hopper if you didn't originally purchase that option.
Also, keep in mind that with your three-day ticket you are eligible for one
Magic Morning at Disneyland Park on either a Tuesday, Thursday, or Saturday. Or, if you happen to be staying at one of the three
Disneyland Resort Hotels, then you have access to
Extra Magic Hour every day of your trip including exclusive access to Disney California Adventure Park on Mondays, Wednesdays, Fridays, and Sundays.
Please visit the Disney Parks Moms Panel again if you need any additional help planning your visit to the Happiest Place on Earth!
-Linda M.