Bomere Heath School in England obviously looked at more than just one of this month's problems, here is what Callen, Annalise,Eddi, Alex and Holly said,

We found out that $7+7+7=21$ so every time you roll a dice it will add up to
seven and when you roll three dice it will add up to $21$.

Noah and Guy wrote,

In three dice you roll three dice and add the top score up and the boottem score together and add it together we found out that the answer was always $21$ thats because three times seven makes $21$ and whatever you get on one side add to the other side always make $7$ like $6$ and $1$ and $2$ and $5$ they both make $7$ .

Sian, Mollie and Liam from Deansfield Community School sent in the following; 

The opposite sides always add up to $7$. When you have $3$ dice and you take the sum of the top and add the sum of the bottom the total equals $21$. This is because $3\times7$ is $21$.
If there are $4$ dice the sum of the top and bottom will always be $28$ because $4\times7=28$

Thank you all for these interesting findings.