Katie, a Production Manager, has been working for a manufacturing company in Kamloops, British Columbia, for 18 years. Claiming to financial difficulties, the company has decided to lay off employees, and Katie's position is being terminated. She is concerned about the termination process and whether her employer is complying with the Employment Standards Act (ESA) of British Columbia. Kindly provide answers for the following: 

1. Katie received only three week's notice of her termination. Under the ESA, is this notice period in compliance, or should she be entitled to a longer notice period based on her length of service?

2. The company has not offered Katie any severance pay, citing financial constraints. According to the ESA, is the company obligated to provide severance pay in this situation, and if so, how is the amount calculated?

3. If the company is terminating group of employees due to the downsizing. Does the ESA outline any specific provisions or requirements for group terminations, and how might
this impact Katie's entitlements?

4. Katie is considering legal action against the company for inadequate notice and lack of severance pay. What obligations does she have under the ESA or other sources of employment law to mitigate her damages, and how might this affect her potential legal claim? Provide a thorough analysis of each issue, referencing relevant sections of the Employment Standards Act of British Columbia and case law.