Three different species with different requirements, they will certainly grow at different rates but it might not mean disaster! Give the cucumber something to climb up, and it will grow vertically out of the way of the other two. Peppers I find pretty fuss free, but they like warm dry conditions, whereas the aubergine is the one likely to get red spider mite if it's too dry. Spraying the aubergine with water once a day might help the humidity around it which would put the mites off. I'd feed the lot once a week minimum as they will all do better if regularly fed, interesting to see what does the best for you!