I am going to switch jobs and the new job will not provide healthcare for 90 days. Do I need to purchase a plan in the meantime to be covered between my old job and new job?
When you get a job you can face up to a 90 day waiting period. What you'll want to do is make sure that you have a plan that counts as Minimum Essential Coverage while you wait for your employer coverage to start. You can then cancel that plan so it ends when your employer coverage begins.
If this gap is the only gap in coverage for the calendar year, you also have the option of going less than three months each year due to a "short coverage gap exemption" everyone qualifies for. This is claimed on the 8965 exemptions form.