Infrared properties of three dimensional gauge theories via supersymmetric indices

The thesis focuses on the study of various supersymmetric three-dimensional gauge theories, mainly with at least N = 3 supersymmetry. We range between very different theories and discuss several different aspects with the aim of validate our assumptions. Therefore, the leitmotiv of this work resides not so much in the topics we cover, but rather in the method that we use to obtain such results. This, in fact, consists in analysing the gauge invariant operators of the theory forming the so-called chiral ring. By having access to the chiral ring structure of the theory and to the operators forming it, we gain insight to the properties that needed to confirm or debunk our hypothesis. We will essentially use two different tools for counting and studying such chiral operators: the Hilbert series and the three-dimensional superconformal index. Thanks to the Hilbert series, we propose a quiver description for the mirror theories of the circle reduction of four-dimensional twisted χ(a2N) theories of class S. These mirrors are, in fact, described by "almost" star-shaped quivers containing both unitary and orthosymplectic gauge groups, along with hypermultiplets in the fundamental representation. On the other hand, by means of the superconformal index, we investigate the N = 2 preserving exactly marginal operators of the so called S-fold theories. In particular, we focus on two families of such theories, constructed by gauging the diagonal flavour symmetry of the T(U(N)) and T[2,12][2,12 ](SU(4)) theories. In addition, we also examine in detail the zero-form and one-form global symmetries of the Aharony-Bergman-Jafferis theories, with at least N = 6 supersymmetry, and with both orthosymplectic and unitary gauge groups. A number of dualities among all these theories are discovered and studied using the aforementioned tools.