Consider a nonlocal boundary value problems of a class of Riemann-Liouville fractional differential equations on infinite intervals. The nonlinear term of the equation is dependent on three lower-order fractional derivative terms, and the boundary value condition is the sum of fractional integral and multipoint boundary value conditions. Firstly, the Green function of the corresponding boundary value problem is constructed, and some properties of it are obtained. Then, the existence and multiplicity of solutions to this boundary value problems are studied by using Leray-Schauder nonlinear alternative theorem, the relevant properties of Green function and monotone iterative method. After sufficient conditions for the existence of unbounded solution and two positive solutions to this boundary value problem are obtained, examples are provided to verify the applicability and feasibility of the main results. The existing academic research findings are generalized by the results.