If uncancelled and used as postage it is worth (surprise) 4 cents.
As a collectible stamp its value will depend entirely on condition, which must be determined by direct observation by an expert in stamps.
4 cents.
The US has issued many dozens of red 3 cent stamps. It will take a bit of work to identify which stamp you are referring to. I would recommend spending some time looking at a US Postage Stamp Catalog to identify the specific stamp. The bonus is that most catalogs will give you an idea of value.
Red cross
2012
The best thing to do is get a copy of Red Book or go to NumisMedia.com because each coin will have to be graded to get a value
Scott #702. Fairly common this 1931 stamp has the minimum value, meaning it is priced based on the cost of cataloging and stocking the item. You can purchase these new or used for 20 cents.
30 cents
The value of one Anna red stamp is 20 cents. It is much higher on Ebay.
That would be Scott number US 1898A. It can be purchased for 20 cents mint and 15 cents in used condition.
This stamp isn't worth a lot. It is Scott Catalog #1036. Issued as a regular issue from 1954 to 1968. 100's of millions were printed. There are 4 values for this stamp and they are:New: 20 centsUsed: 20 centsFace: 4 centsDealer: 3 cents(Scott Specialized Catalog of US Stamps 2011, page 104)
The US has issued dozens of George Washington stamps, many with a value of 2 cents. You will have to completely identify the stamp to find a value. A catalog, available in most libraries can help you determine the value. Chances are a dealer won't purchase a single stamp unless it is worth more than a dollar or two.
This is part of the Washington bicentennial issue of 1932. A dealer would charge 10 to 25 cents for it , mint or used.
All US cents since 1909 have been Lincoln cents. The older coins may be more valuable based on their scarcity and condition, but no coin has a guaranteed value above its face value, which here is one cent. Lincoln Cents are the most collected series of coins collected in the entire world. You can easily complie a complete collection, in circulated grades, of all but the most rare dates and mintmark combinations. The value of Lincoln Cents range from 1 cent, for common pocket change, to over $200,000 for a single Red, Brilliant Uncirculated 1922, with no D cent.