This function will create a ggplot2 figure with propensity scores on the
x-axis and the outcome on the y-axis. Three Loess regression lines will be plotted
based upon the propensity scores from model. Since each model produces
propensity scores for two of the three groups, the propensity score for the third
group in each matched triplet will be the mean of the other two. If model
is not specified, the default will be to use the model that estimates the propensity
scores for the first two groups in the matching order.