All blank negatives except for three pictures?

SUCKAHC

New member
I just started using an Asahi Pentax k1000, and I went to get my film developed and it turns out only three pictures came out. I checked the negatives and everything was blank besides the three. Why? I don't know if this may have caused it, but I frequently change lenses in the middle of the roll, but I heard the mirror blocks the light? Is this true?

Thank you!
 
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