Hi there Eugene,
Welcome to planDisney and thank you so much for your question!
According to the
eTicket Terms & Conditions page, standard
theme park tickets must be used within 13 days of first use. However, if your plans have changed and you're no longer able to use all three days then you will likely be able to use the value of the expired third day toward the purchase of future theme park tickets, again if you have standard theme park tickets.
If you know the details of your trip have changed but have already purchase your theme park tickets then I would recommend
calling a Cast Member at the Tickets line. They will be able to see the specific details of your situation and be able to let you know exactly what to expect and what options you may have.
In the meantime, as you continue to plan your trip, make sure to check out the
Disney Parks Blog! There are always so many fun things happening around the Happiest Place on Earth and I've found the Disney Parks Blog is the best way to keep up with it all.
Thanks again for your question, Eugene! I hope you have a wonderful visit to the Disneyland Resort.
Have a Magical Day,
Greg