Are dental dams effective against transmitting an std?

To some extent. Yes, to some extent. Latex in between any skin and other infected skin or secretions will help. However, if it doesnt cover all skin or prevent infected secretions from seeping around the latex to reach your skin, then it wont help. Helpful, but far from 100 % effective.
No. A dental dam serves to isolate the work area from contamination, from the wetting of saliva, and from the moisture of the breath interfering certain bonding process. It also aid in using abrasive particle during air abrasion, preventing swallow of sharp object, and keep the bleeding away from the working area. They are not used for preventing std transmission.
Not the best. A dental dam may help since it is a barrier, but it's really not made for this. Get the disease taken care of in the case of curable ones or stay away from sex with individuals infected with non-curable ones is the best advice.