Can someone explain to me how to file taxes if I had Canadian income for 3 months and US income for the rest of the year?

Three things to keep in mind when filing foreign income in a Canadian tax return:

1. Residency Status – applied at the beginning of your tax return. - How do I determine my residency status? 
2. Type of Income earned - How do I report foreign income, foreign pension and other foreign amounts? 
3. Country of Origin to determine if you get a Foreign tax Credit back based on the tax treaty we have with the country in question (if any). In this case, it is the USA and we do have a tax treaty with them which means you will avoid double taxation. To find Foreign Tax Credit form  click on the “Find” icon at the top right corner of the screen and type in “Foreign Tax Credit”.  How Are Taxes Assessed for U.S. Citizens Working in Canada?

Also please review the following link in case you have Foreign Property over $100,000 outside of Canada, you may need to fill out a T1135 form which can be found under the “Income” tab  - "Other Income sources" of your tax return. What You Need to Know About Cross-Border Taxation