Gasification has been a successful technology used to convert coal to synthesis gas to produce hydrogen, fuels, and chemicals. The approach of using mixtures of coal, biomass, and plastic in high-pressure, entrain flow gasification (EFG) is a promising method to generate hydrogen through beneficial feedstocks with the potential for net negative carbon dioxide emissions. Despite EFG being a well-researched technology, further investigation is necessary to achieve reliable feed and operation of such mixtures as there are no established means to do so. The objective of this work is to evaluate the feasibility of coal-biomass-plastic slurries for high-pressure EFG. To assess flowability and separation patterns, the mixtures are evaluated for viscosity and phase behavior. A rotational viscometer was used to evaluate viscosity as a function of shear rate and temperature. Shear thinning behavior was apparent in all mixtures with the viscosity being heavily correlated to the coal concentration when above 25 wt%. In addition, viscosity decreased with an increasing shear rate but also with time at a constant shear rate. Increasing plastic oil decreased the viscosity by up to 20%. However, phase separation was present with plastic oil concentrations over 20wt%. Regardless of composition, the viscosity as a function of temperature followed an Arrhenius-type relationship. Thermogravimetric analysis of the blended slurries is used to determine the moisture content, volatiles, and fixed carbon with ash as the residual. Additionally, it is a means to quantify the degree of mixture separation as the composition of separated phases. The favorable mixtures were successfully fed at roughly one ton per day with a progressive cavity pump at pressures greater than 300 psi for four hours. If separation and mixture instability during pumping is prevented, continuous high pressure of coal-biomass-plastic mixtures has been demonstrated.