I am making a blog about this because I searched for an hour and tried many things before figuring this out by myself. It might save someone out there time in the future.
After uninstalling windows 10 - on the task bar the calculator, windows explorer and internet explorer icons were missing and all that appeared were blank white squares. Clicking them gave a warning that the item was missing and asked if I wanted to remove it. I didn't because I wasn't sure what that might do.
Finally I went through the start button, typed in each item, like calculator, AND when it came up I right clicked the item to see it gave the option to unpin it from the task bar. I did. Then I re-pinned it. VOILA - back on the task bar for all three items.
WHEW FIXED!
I don't like 10 and I'm sure if I live long enough I'll have to use 10 but I'll fight it!