There are a number of reasons why a dragon wont poo (as often as we'd like or as regularly).
Can be dietary, due to insects like superworms and mealworms be a large part of the diet, due to ingesting some substrate which has created a blockage, metabolism, temperatures and lack of solid food and lack of protein.
Firstly how long has it been since your dragon last did a poo ? .... OP says a few days
and is it regular ? .... OP says used to be pooing every day
A few days between poos for a larger juvenile or adult beardie is not an issue , some have very slow metabolism and save up their poo for up to 14 days , I don't start intervening ( with oral doses of olive oil until it's been 7 days, and then baths with tummy rubs from 10 days.
For a very small / young dragon a few days is common, I would start intervening at 4 days with oral doses of olive oil and natural gentle laxative , and
bathing with tummy rubs from days.
At 3 months old he's still only a hatchling , young juvenile.
Do you feel any hard lumps in it's tummy ?
Will be helpful to see photos of the dragon , and it's viv and lighting (show how everything is set up, show the substrate).
Need details of the UV and lights, and temperatures.
About his diet , there are issues.
Your 3 month old should be on a nearly entirely live insect protein diet with veg offered , but mostly eating live insects , 2 or 3 times per day.
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