Hello,
the design of the M 3 viewfinder is different from all later types, beginning with the M 2. Therefore all later viewfinders till the MP/M 7 are more or less prone to flare. I had exchanged my M 6 viewfinder window against the newer multicoated MP type at the LCS, it was very expensive, now I have much less flare from light sources outside the viewing frame but now instead of that the two LED's of the lightmeter are so bright that it is difficult to see correctly the balance of brightness. Maybe that is not the case in an original MP. Besides: My M 3 DS viewfinder has a very slight yellowish tint. Do not try to dissassemble a M 3 viewfinder as the prism could separate and no spare parts are available. Even the LCS does not repair or clean a M 3 viewfinder except at your own risk.