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Purpose: Two‐dimensional GRID therapy, traditionally planned and delivered using a dedicated GRID block or MLC modulation, has shown clinical efficacy in treating bulky tumors. However, the large dose to normal tissues outside target can be limiting. We hypothesize that modulation in the third dimension will improve dose sparing of normal tissues, maximize the bystander effect within the target, and ultimately improve the therapy effectiveness. This study aims to investigate the feasibility of a three‐dimensional GRID technique using conventional LINACs to achieve a 3D lattice of high dose volumes within a target. Methods: Datasets of patient's having large tumor sizes were used to investigate the planning and delivering of 3D GRID using a Varian TrueBeam linac. Original patient contours of PTV are exported from a TPS to DICOManTX where 3D GRID targets are generated in programmable configurations. A structure of avoidance (SOA), i.e., PTV minus GRID targets, is also generated to facilitate inverse planning to achieve the desired pattern. The artificial structures were sent back to the TPS where an IMRT or VMAT plan is designed to deliver a desired high dose to GRID targets while minimizing the dose to the SOA as much as possible. Results: The programmable GRID target generator enables us to modify the target geometry to maximize the peak‐to‐valley ratio. Preliminary results show that plans based on spherical GRID targets achieve a higher peak‐to‐valley dose ratio compared with cylindrical targets. High dose spillage outside the target was eliminated. IMRT planning requires the number of beams to be larger than 16, while for VMAT the number of arcs should be at least 4 in order to achieve dosimetric goals. Conclusion: Planning and delivering 3D GRID therapy using conventional LINACs was shown to be feasible. More research and development are required before this new modality can be implemented clinically.