We could also assume that the three ships were there only as a last-ditch defense of Mars, not of the entire Sol system. After all,
a) the Sol system is a big place to be defended, and a defense force launched from Mars would be impractical in stopping a threat that came from any other direction
Indeed. If we go by the assumption that a season covers an entire calendar year from January 1 to December 31, then BOBW Pt. 2 (stardate 44001.4) would've taken place on or about January 1, 2367. On that date, according to the Celestia simulator, Mars will be on the opposite side of the Earth from Wolf 359. So the Borg would reach Earth before they reached Mars. In fact, there's no time between about October '65 and September '67 when Mars would be more or less directly between Wolf 359 and Earth.
b) the Borg took special care not to fly to Earth straight away, but to visit all the inhabited planets in the system (we saw Saturn and Mars and heard over the comms that Jupiter had been attacked) before settling on Earth orbit, so close-in defenses specific to Mars would have been forced to respond.
That's right. I'm looking at my Celestia display for January 2367, and it's oriented so that Wolf 359 is way off to the left (at 9 o'clock). Treating Earth as the center, Mars is at about 2:30 to 3 o'clock, Jupiter's roughly the same direction but farther away, and Saturn is at 7 o'clock, but about 10 times farther than Mars. This could work if the cube followed a spiral course around the Sun, first getting a bit of a gravity assist from Saturn to aim it toward Jupiter, then looping about 90-120 degrees around Jupiter to get on course for Mars and Earth, coming in from just about exactly the opposite direction from Wolf 359.
But the question is, why would the Borg bother to do that instead of going directly to Earth? Did they specifically
want to take out the defenses at Saturn, Jupiter, and Mars before tackling Earth itself? Were they figuring that most of Sol System's defenses would've been between Earth and Wolf 359, so they looped around the back to take us by surprise? Why would they have bothered, when we had nothing that could pose a threat to them?
Of course, the real explanation is that fiction writers always assume that the planets are in a straight line. Bad enough they almost never think 3-dimensionally about space, this is a case where people usually forget to think even 2-dimensionally.