I have huge list of keywords contains 1 word to 5 words in a cell..

The keywords that having only 3 words should remain in the cell and others has to be removed

Ex. This is the actual keywords I have
doctor ps2
doctor salary
doctor clipart
doctor government agencies
doctor government grants
doctor homeland security
doctor thesaurus
doctor who
doctor wii
doctor post office jobs
doctor survival kit

It should like this after trim
doctor government agencies
doctor government grants
doctor homeland security
doctor survival kit

You can see the above having only 3 words in a cell remains and others are removed, how can I do this in Excel?

Thanks for the support.