I calculate your distribution losses at 13 dB based on 50' of RG-6 from the antenna to the distribution point (3 dB), a 4-way passive splitter (8 dB) and another 30' of RG-6 (2 dB). If you have only three tuners connected, a 3-way splitter will have even less loss. Most 3-way splitters have a -4 dB port and two -8 dB ports. There a a few 3-way splitters that have equal output on all ports. Example:
http://www.antennasdirect.com/store/...tter-SPT3.html
Either of the the preamplifiers suggested thus far have enough gain to overcome all of that loss.
Every amplifier is a source of noise over and above the noise received by the antenna. A good consumer grade distribution amplifier has a noise figure in the 3.0 dB neighborhood. Why go to some effort and expense to eliminate noise at the preamp if you're going to add a significant amount 50' later?
As an example, let's say the antenna receives a signal that is 30 dB stronger than the noise (there is a signal to noise ratio, SNR of 30 dB). After passing through a preamplifier with a gain of 24 dB and a NF of 0.4 dB, the desired signal power is 24 dB higher... but the noise is 24.4 dB higher, which means that the signal to noise ratio at the output of the preamp is 29.6 dB. If you delivered that signal to a distribution amplifier with a NF of 3.0 dB, the signal at the output of the DA would have a SNR of 26.6 dB.
The bottom line is, you can get all the gain you need from one amplifier. Adding a second needlessly elevates the noise delivered to the tuner.