At 180 bpm, this means that a metronome needs to be set at 180 and each click will represent one beat. Your rhythm pattern is (I hope) 1-2-3and -4and as in 2 crotchets(1/4) followed by 4 quavers (1/8) notes, or strums.If you count the 1,2,3,4 evenly, the 'and' will go between. The count is in time with the metronome.
After that, your theory goes awry. There are 3 bps, so a whole bar of 4/4 will take 1.33 secs. Divide by the 4 beats, and a downstrum will take 0.3325 of a second, except where there's an upstrum as well, where BOTH will take 0.3325s. So an upstrum will then be 0.166s long.I'm fascinated as to how you're going to measure this, though.
The bpm is there to establish a pulse. Imagine a band shouting "1,2,1234" at the start of a song. The 1234 is the count which can be quoted in bpm. The pulse of the song then goes at that tempo (hopefully!).
ON REFLECTION-
If, instead, there are 3 beats to the bar, it will look like 2 crotchets and 4 semiquavers.This is different, in that one bar will take exactly 1 sec., and the last four strums will take 0.08333sec each.
Whichever strum pattern it is, there's hardly any time to change chord at the end of a bar !
EDIT - it looks from your edit that there are 4 beats in the bar, and the strum consists of crotchet, crotchet, and four quavers.As in my first case scenario.