Waiting for 3 consecutive miscarriages before doing any testing is common practice, not that it's right, but that's what docs do. You could pay for the progesterone test yourself. There are online labs where you can print out your own lab orders, take them to a participating lab and have the test run yourself. You have to pay in full because your medical insurance will almost certainly not cover it without a doc's order but a progesterone test doesn't cost very much. The results are given directly to you, too, so you don't have wait for them to be sent to a doctor and then wait for the doc's office to call you.
Here's just one I found when I googled online blood testing labs:
http://www.personalabs.com/howitworks.php. I can't attest to that one because I've never used it but you can see that they are out there and you can do this yourself. You could also go ahead and start using progesterone cream. However, it probably won't make a difference. I was using it when I got pg in July of this year and still miscarried. The MFM doc I was seeing tested my progesterone levels while I was still using the cream but after I had already started miscarrying and my level was only 1.2 or something ridiculously low like that. Obviously, the progesterone cream was doing nothing to raise my progesterone levels.
The other thing to do, as frustrating as it is, is to keep searching until you find a HCP who will support you. When I got pg in May 2009 I finally convinced the OB I was seeing to check my progesterone. I had already had 3 or 4 first trimester miscarriages and been tested for an array of blood clotting and auto-immune disorders as well as having all the hormone level tests and had been prescribed progesterone suppositories for 2 of my 3 previous full term pgs. Even with all that the doc was reluctant. So, we got it tested once around 7dpo. It was 14.2 or something like that. He said that was good, wouldn't prescribe progesterone and that was that. The baby died at around 8w and I miscarried at 11w.
The problem is that there is no definitive evidence that progesterone supplementation helps once a pg has been established. So some (maybe most) docs won't prescribe it. I don't see why it's such a big deal because it's extremely safe on the scale of meds during pg and very inexpensive, relatively.
HTH