I have an order for 100 seeds coming. Being that they need to be three feet apart, how large should my bed be?

Don't know what you're planting, but a bed 6 feet wide by about 100 feet long should cover it. 3 seeds to a row and 33 rows would plant 99 seeds. Or you can make the bed wider and not as long. For example 12' x 60'.

Looks to me like if you made your rows 100 ft. long, you could plant 33 seed to a 100 ft. row and you would only need 3 rows if you plant your seed 3ft. apart.. But I don't see why your seed needs to be 3 ft. apart. If your planting seed, 1 ft. would be more than enough and you could probably get by with 6 inches apart. You will probably have some seeds die on you.
Next question is, when are your seed coming? If your planting seng seeds, you should be buying seed that have been stratified. These seed would need to be planted in the fall of the year. That would preferably be in mid to late October. If your buying non stratified seed your taking a bigger risk of not seeing your seed ever grow.

Don, my seeds are stratified from Jeff here in the Trap Shed.