Dengue fever, Zika, and chikungunya are arboviral infection diseases transmitted mainly by two vectors: Aedes aegypti and Aedes albopictus. Between 2015 and 2016 Rio de Janeiro experienced a joint epidemic of the three diseases. As the diseases are transmitted by the same vectors and symptons are similar, there is uncertainty about the number of cases registered. We propose a spatio-temporal hierarchical model that assumes a Poisson model for the total number of cases of arboviral infection diseases for each week and neighborhood and, conditioned on the total number of cases, we assume a multinomial model for the three diseases. We explore different structures for the log-relative risk of the total number of cases and, the parameters of the multinomial distribution, to learn how the three diseases are correlated across space, time and among themselves. Inference is performed under the Bayesian framework.