Question about getting average value

If I have a PI equation on getting average value like this one

avg(tagval('tagA','y'), (tagval('tagB','y-1'), (tagval('tagC','y-2'))

but I want to exclude those tags having less than a given value (e.g. -20 deg celsius).  How can I insert this filtering in the above equation?  Any help will be appreciated.

Thanks OsIsoft!