This one needs to be done on a track or somewhere where 200s or 1/8 miles are marked out. I call it the Random 3. Warm up and cool down as usual, but the workout proceeds as follows:
The workout consists of 200s, 400s, and 600s. The length of the next interval is determined randomly by the hundredths digit on your watch when you press stop/lap/split. If that digit is...
1, 2, or 3, then your next interval is a 200
4, 5, or 6, then your next interval is a 400
7, 8, or 9, then your next interval is a 600
0, then you use the tenths digit, if the tenths digit is also 0, then use the seconds digit, and so on, until you determine the length your next interval. (To determine your first interval, you should stop your watch near the end of your warm up.)
The rest between intervals is 200 and you should do the intervals faster than 5K pace. Keep doing them until you've run a total of 3 miles (12 laps on the track), and that includes the rest 200s. You must finish the 12th lap running hard, so if your random draws leave you with the last 200 as rest, then run it hard anyway. You might get stuck doing an 800 for your last interval.
The last time I did this, I drew the following:
600 (+200 rest = 2 laps complete)
600 (+200 rest = 4 laps complete)
400 (+200 rest = 5.5 laps complete)
400 (+200 rest = 7 laps complete)
200 (+200 rest = 8 laps complete)
600 (+200 rest = 10 laps complete)
400 (+200 rest = 11.5 laps complete)
200 (= 12 laps complete)
The uncertainty adds an element of fun that makes it go by quickly. This one is fun to do with a partner.