When Do I Start

I was just wondering when should I start to introduce grow nutes to my plants, they have sprouted up and in about 3 days I will be transplanting them to a mini HB, Just need to know when to start to do 1/4 str nutes

at 3 weeks

3 weeks can be a good rule of thumb... but it can depend on their environment. If you have minimal lighting for example, then your plants may still be too small for the nutes. Different strains grow at different rates based on their conditions. if you are unsure about your conditons (including temp, relative humidity, ect...) then I'd say wait until the 3rd or 4th node.