No crank

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just thought I would ask for one last bit of help before I just cut my losses and get a mechanic out to have a look and fix it.
so I got my multi meter out again and took fuse 1 out (buzzing stopped) and I put my negative lead on the negative batter post and the positive on one side of where the fuse goes in and it was getting the 12v (which I assume is normal) I then put it on the other side and it was getting voltage as well, it was round about 3v. I have been trying to find out if this is normal or not or if there is maybe something back feeding power into it? anymore help would be really appreciated

Perfectly normal depending on where the output side of the fuse goes to.
Many times you will see low voltages on unpowered fuses - most folk wouldn't see these voltages as they run through a fusebox with a testlight.
The buzzing is telling you that something somewhere is not getting a good voltage or ground.

Ah I see, was hopping I could have started to narrow it down with that but maybe not
I managed to get someone out with a diagnostic machine but it wouldn't connect to car, fuses were checked again and we're all fine so not sure why it wouldn't work, any ideas?

I have been checking a few more things, when I disconnect a connector from the ecu the buzzing stops, could this be where the problem lies? (Burnt solder)

i think i may have found my problem today, i watched one of Scanner Danner's videos on how to test for a bad computer which was very helpful as the car had a no crank as well. so i tested all the sensors and they weren't getting the correct voltage (were getting just over 2v) so once i had tested them all they were the exact same. so i unplugged the ECU and could see that it had been opened before as the face plate was bent at the edges, so i opened it up again using the correct tools and had a quick look. At first i couldn't see any burnt solders until i was about to close it up and looked on the face plate which had a burn mark on it, so i looked on the board where this was and i spotted it. There is a burnt solder and the actual bored around it there is a small burn mark on as well. i am going to get the ecu sent away and rebuilt but i was just wondering what causes this to happen so that when i connect it again the same thing doesn't happen. also i have an amp and subwoofer in the car, could this have spiked the voltage fried it?