I follow the same standards for level 3 as in my online program Math & Logic Puzzles.
Here are a couple of my favorites. You can download the puzzles below, retrieved from dobmaths that has a lot of good material. I will give 1 puzzle and also explain the solution and strategies for solving.
Symbol Sums: Excellent puzzle to train 2 equations with 2 unknowns. Sometimes it is easy to use trial and error and guess the numbers if one equation adds up to a very low number, say 5 (where the only option for a 4 x 4 grid is 1+1+1+2). However, with higher numbers it gets more complicated and the solver has to show math skills.
Octa-Fill: A good mix between (simple) math and use of the 3 strategies I outlined here. See the solutions for how I used these strategies to solve the puzzles.
Symbol Sums – Solution (coming soon)
Octa-Fill – Solution (coming soon)
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