Transiting hot gas giant exoplanets are great targets for atmospheric investigations due to their extended atmospheres and their strong obscuring effect on the observed starlight when they pass in front of their host star. Transmission spectroscopy can probe transiting exoplanets at various wavelengths and, hence, reveal the dominant opacity sources in specific regions of their atmospheres. In this thesis, I present groundbased transmission spectroscopy obtained in the optical for three exoplanets and describe the various analysis methods explored in order to produce the characteristic transmission spectrum for each exoplanet. The first target was the test case WASP-75b for which a rudimentary data reduction and analysis procedure resulted in a transmission spectrum of a low precision. The data set for this target was mainly used for practice purposes and so the analysis is brief. The next target was the low-density hot Saturn WASP-88b. Here, I examined a polynomial detrending approach and a detrending approach based on Gaussian Processes (GPs) in an effort to isolate the systematic effects from the transit light curves. The resulting transmission spectrum was found to be featureless with atmospheric models indicating the presence of high-altitude haze. The final target was the hot Jupiter WASP-74b. In this case, a new method that performs a common-mode correction only to the light curves of the target and considers an exponential of airmass during GP detrending was also presented in the spectroscopic analysis. This method resulted in increased transit depth precision and a steep scattering slope in the measured blue-optical transmission spectrum that suggests enhanced haze in the upper atmosphere. The results for WASP-88b and WASP-74b are representative of an emerging trend of steep scattering slopes observed in an increasing number of exoplanet atmospheres. The physics behind this spectral shape is not yet well-understood but several mechanisms have been proposed
