How rare is it to get 3 home runs and a triple in one game?

It just happened last night for the second time in 50 years. Fred Lynn did it in 1975, and last night, it was Ryan Braun. (At Petco Park, where it's unusually hard "early in the season when the 'marine layer' off the Pacific Ocean envelops the spacious facility in a thick, damp shroud." In fact, Braun was the first person to hit 3 home runs in one game at Petco Park.)

It would be better to get 4 home runs, but it's much more unusual to get 3 home runs and a triple. Triples are much rarer than home runs. And — did you realize? — the triple has become rarer over time:

With the exception of the almost extinct inside-the-park home run, the triple is rarest of hits. This was not always So. For more than fifty years after the founding of the major leagues, the home run was the rarest hit, followed by the triple, double, and single. The logic behind this was obvious: The farther a batter struck the ball, the more bases he could reach. 

Even such changes as overhand pitching and enclosed ballparks did nothing to affect the natural order of hits. From 1901 to 1929 the average distribution was: 76.9 percent for singles, 15.2 percent for doubles, 5.3 percent for triples, and 2.7 percent for homers. In the pre-Ruthian years, there were roughly three to four times as many triples as homers. Furthermore, the overwhelming majority of everyday players who ended their careers before 1930 had more three-base hits than home runs. This list includes Home Run Baker, Honus Wagner, Ty Cobb, Joe Jackson, Eddie Collins, Tris Speaker, George Sisler, and Sam Crawford; the latter holds the career record for triples with 312. 

The heyday of the three-base hit was the nineteen-teens. The newly built concrete-and-steel parks had huge outfields and distant fences, with foul lines often in the 370-foot range and with center-field fences more than 450 feet away. Although a new ball was used after 1910, it was dead by modern standards and often doctored by the pitchers, so outfielders played shallow. Balls hit over their heads or line drives in the gaps (especially in the early innings before the ball got soft) could roll to the deepest part of the park. With pitching dominant and low scores common, the strategy of the times was that it was often worth the risk of stretching a double in order to get one base closer to home.... 

The emergence of the home run in the 1920s was the death knell of the three-base hit. This event was no accident, but a conscious effort by the team owners noting the correlation between the increase in home runs and the rise in attendance. The baseball establishment assisted the triples-to-homer shift in two significant ways. First, the architecture of the ballparks was changed. The outfield fences were moved in, shortening the distance for a home run and reducing the length of outfielders' throws to third. The second alteration was the ball. In the 1920s it was given a more resilient center and many more new balls were used per game. When the "rabbit ball" was introduced in 1930, batting averages and home runs skyrocketed. 

By 1920 the ratio of triples to home runs had dropped from three or four to one to only two to one. By the late 1920s triples and home runs were virtually even. In 1929 home runs surpassed triples for the first time. Over the years the gap has widened. In the late 1940s there were about 2.5 home runs for every triple. A decade later that ratio was more than three to one, and by 1988 there were almost four round trippers for every three-base hit – a level of domination the triple never enjoyed over the homer.

More at the link.