There are many sites offering free printable third grade maths worksheets for free, though not so many deal with estimation problems.
One site that does is math-aids.com/Estimation, which offers 15 worksheets to download for free.
You can find another 24 free worksheets at education.com/worksheets/third-grade/rounding-estimation, these deal more with rounding numbers up, to help a child make telling the time or adding up in their head a bit easier, when it isn't important to deal with exact numbers.
If those aren't suitable, you can find more third grade level estimation problems at mathblaster.com/.../math-worksheets and mathworksheets4kids.com.
If the kids you teach are anything like these guys, they'll soon be singing about the wonders of estimating numbers!
