These herbs are traditionally used by native American women to facilitate childbirth, tone the uterus and strengthen contractions. There may be some side effects from the cohoshes in some women. Pennyroyal is also used as an abortive in early pregnancy, but in this combination the dose will probably not be high enough.
First of all, birth is a natural process your body is perfectly prepared for. You can do your part by eating well, being fit and trusting you body to do its job. If you want, have helpers/doula/midwife who are as confident about your ability as you. Learn about active birth, unlearn all about fear, stress and giving up your body to doctors etc. unless really necessary.
Most interventions are unnecessary, mindset and your freedom to move your body into whatever position you instinctively feel is "right" are key. Gravity and relaxation are your allies. Birth is beautiful!
Herbs and homeopathy, as well as water (I had a water birth for my first baby-wonderful!) can be a great help, but choose them wisely and for your specific situation.
Raspberry Leaf tea can be drunk 3 times a day during pregnancy, labor and breastfeeding, and is a great nourishing tonic, and has no side effects. Tea is much better than capsules, as it gets absorbed better.
Another way than as tincture is to take blue and black cohosh as a homeopathic preparation- the names are Caulophyllum and Cimicifuga. You could take a 30C potency and alternate both remedies 2-3 times a day.
You can find lots of useful information in the midwife archives at http://www.gentlebirth.org.
A herbal book I use is Susun Weed's "Wise woman herbal for the childbearing year". For homeopathy, you could look at Miranda Castro's "Homeopathy for Pregnancy, Birth, and your Baby's First Year". It's always good to know what you're doing when self-medicating.