The Value Of Multiple Stamps On Same Cover. Help Needed!

How would one determine the value of a cover, if there are three stamps on the cover? Suppose, for example, there are three #20's on the same cover and one is XF and the other two are VF. How is the value of the cover determined? The Scott catalog does not give a CV for three #20's on cover. Do you determine the value of the cover by treating each stamp individually on the cover? Does the value of being on-cover get added to each individual stamp? How is it done? Well, here's what I'm working with:

There really is no exact formula for determining the value of a cover, especially without a picture. You don't clarify if they are a strip or 3 singles. What condition is the cover in? Is it a scarce postmark? Is it addressed to anyone well known? The plating of the stamps could skyrocket the value if they are well sought after positions.
A picture is worth a thousand guesses. If they are 3 #20's then I would think the cover is worth good money.

Thanks, Stallzer, I've updated the post to include pictures.

For a "standard" cover like this (normal domestic first class rate and usage), it is probably best to start with the basic cover value and add premiums or take discounts as deserved. And remember that Scott value is a retail price based on a Very Fine stamp. Collectors would then expect a Very Fine cover overall.

This certainly has enough value that plating the stamps (if you are able to do so) might help the value. It isn't so rare that you couldn't find a price range for similar covers online.

Would they be worth more off-cover or on-cover?

On cover by a long way, removing them from cover would be sac religious. Determining the plate and positions is needed. I'm on my iPhone now so can't get a good look at them. If nobody has plated them I will try over the weekend.

I see a cover on ebay with a strip of 3 that sold for $147.00, I will post a link later.

The stamp on left looks the best. Perhaps VF centering for issue at best.
Other two stamps are not centered well and have obvious issues with perfs on top and straight edge for stamp on right.
This is worth more on cover than if you tried to sell stamps individually.
Keep it as is!!

Here's Mr Davis again