[sllabres]

I think only the Grand Tour program was possible every 175 years: From Wikipedia [1]: "that an alignment of Jupiter, Saturn, Uranus, and Neptune that would occur in the late 1970s would enable a single spacecraft to visit all of the outer planets by using gravity assists."

Gravity assists with more than one planet are more frequent. Cassini-Huygens [2] as example had five (Venus, Venus, Earth, Jupiter, Saturn)

I would suspect when the goal ist only to leave the solar system as fast as possible (and don't reach a specific planet) they are much more often.

[1] https://en.wikipedia.org/wiki/Grand_Tour_program [2] https://en.wikipedia.org/wiki/Cassini%E2%80%93Huygens