Probability is the study of determining the chances of an event being successful or not. To determine the probability of, say, 5 events happening, you would have to add their individual probabilities. For the probability of household having less than 3 televisions, you add the data for 2, 1 and 0 televisions.
Total Probability = 3% + 11% + 28% = 42% (or 0.42 if in decimal form)