Booleans: Functions that get same results using != and ==

I am experimenting with using different booleans to get the same results. I have two functions and I don't understand why one works and the other doesn't.

This works:

dna='ATGTGGTAG'

def orf_4(dna):
    if dna[len(dna)-3:]=='TGA' or dna[len(dna)-3:]== 'TAG' or dna[len(dna)-3:]=='TAA':
        return 'This could be an ORF'
    else:
        return 'This is not an ORF'

print orf_4(dna)

This does not. It returns 'This is not an ORF' for any input.

dna='ATGTGGTAG'

def orf_3(dna):
    if dna[len(dna)-3:]!='TGA' or dna[len(dna)-3:]!= 'TAG' or dna[len(dna)-3:]!='TAA':
        return 'This is not an ORF'
    else:
        return 'This could be an ORF' 
print orf_3(dna)

Does anybody have a suggestion as to why this is?