Yes, mesh3 cards can be used to measure dosage in concrete. These cards are specifically designed for use in MCNP simulations and can accurately calculate the radiation dose in different materials, including concrete. It is possible that your friend may have been referring to mesh1 cards, which are also commonly used in MCNP simulations. However, these cards are better suited for measuring neutron fluence rather than radiation dose. If you are unsure about which card to use, it is best to consult with a knowledgeable individual or refer to examples online. There are many resources available that provide examples of using tmesh cards in MCNP simulations for measuring dosage in concrete.