First off, I really don’t think you need to inject EOD – that seems like overkill, but if you’re gonna shoot for 3 days per week for 100mg/week, here’s what I’d do (and have routinely done):
Assuming you have the more common 200mg/ml Test Cyp, which is what I have, I just back-fill a little insulin syringe to the .5, which is your 100mg. Then I use the same syringe for either 2 or 3 injections (sub-cutaneous) during the week. Just roughly inject 1/3 each time. It is not at all necessary to “get it right” at exactly 1/3.
As long as you keep yourself clean, and you can even wipe the needle with an alcohol wipe if you’re paranoid, you should be completely fine. It’s absurd that one can’t use the same syringe a couple of times on himself–it isn’t like you’re “sharing needles” with someone else.