There are 2 activities. Both go with standards:
MCC3.1.NBT.2 Fluently add and subtract within 1000 using strategies and algorithms based on place value, properties of operations, and/or the relationship between addition and subtraction.
3.OA.8 Solve word problems using currently mastered operations.
In the first activity students identify what is missing (either a part or the whole) and solve for it. I had them put a "?" for the unknown and write an equation as well.
In the second activity, students write their own word problems that can be solved using a part-part-whole model. Challenge models are also provided on the back for students who are able to write more complex problems.