If your period is 42 days late then you have not missed 3 periods.
If your normal cycle is every 28 days or so, you are around 2 and half weeks late. You may have done the home tests too early and that is probably why the home testing kits showed negative results.
If you are 42 days late then you will have missed 1 period.
The reason for a repeat of a blood test would normally be that the results were not clear enough to give a correct reading.
There are other reasons and not just pregnancy for missing periods
What happened with your blood test? Can you go back in to get another one? I'm sorry I'm being of no help and I wish I could tell you what's going on but I'm not sure. It can take some women a long time to get a positive on a home pregnancy test. The blood test is more accurate so if you can get in to have it redone that may give you some answers.