The number of blades does not change the effective thrust by any noticeable amount, provided the diameter and RPM stay the same. A smaller diameter will result in less thrust, which may be necessary when using more blades, to keep the engine near peak torque RPM. If you can find a propeller with blades that are 2/3 the width of the two blade prop, you can use the same diameter and may even see a small increase in thrust, maybe only 1 or 2%. Here's a couple of reports on the subject

http://naca.larc.nasa.gov/reports/1938/naca-report-640/
http://naca.larc.nasa.gov/reports/1939/naca-tn-698/