Calcium deficiency or something else?

You've got yourself a major calcium deficiency going here, buddy... and just feeding it calcium or cal/mag is not going to help, particularly since you say you've been doing that and it getting worse. Your diary doesn't mention the PH you maintain with your plant and I think this is the critical source of your problem - and needs to be corrected asap! Calcium is only absorbed when the PH is 6.2 or higher - so if your PH is too low, you can feed it pure calicum and tons of it and it won't absorb one drop... The magnitude of this, and the coloring of the leaves tell me that your PH is far, FAR too low and you need to bring it up. If you don't have one, invest in a PH pen so you can measure the PH of everything you give her - water, nutes... that range should be between 6.2-6.5... The other source of your problem, I think, is that you're relying on MiracleGro which is NOT a cannabis-friendly nute... she needs to be on some bloom nutes with calmag given BUT ONLY ONCE A WEEK... Pick a decent line of nutes - I use FoxFarm, others you might consider are Advanced Nutrients... you don't have to buy the whole set - just get the bloom nutes for now (and if you choose Fox Farm, buy a small bottle of Tiger Bloom - it will give you a small bit of nitrogen, and a lot of Phosphorous and Potassium ... ideal.... get just a small bottle for now as you only want to get this grow through to harvest) and save the MiracleGro for houseplants... Once you have your PH pen (and DON'T rely on strips or drips because they're WILDLY inaccurate), you're going to want to flush your plant by running enough PH'd water through her to equal 3x the pot size... You're growing in a 3 gallon so you'll want to run 9 gallons of ph'd water through each pot. Add about 1/2 strength dose of bloom nutes (if you choose Fox Farm - and I don't have any investment in that brand save for it's the one I use - I would add 1.5tsp of Tiger Bloom) to the last gallon of water - and then sit back and let her dry out before resuming your water/feed/water/feed schedule... But always, Always ALWAYS make sure what you're giving her is in that PH range... One more thing to note - leaves that are already damaged will NOT get better - so to tell whether you've conquered the problem or not, look to the new growth... Good luck!