A group of workers can plant at a rate of 30 acres per day.

To find out how many acres the workers can plant in 3 days, you can multiply the planting rate by the number of days:

\[ \text{Total acres planted} = \text{Planting rate} \times \text{Number of days} \]

Substituting the values:

\[ \text{Total acres planted} = 30 , \text{acres/day} \times 3 , \text{days} = 90 , \text{acres} \]

So, they can plant 90 acres in 3 days.