Thank you for your email. I understand how difficult it is managing your twin boys.
Your younger boys are instinctively competing for everything from attention to space and as they do not have the vocabulary yet to express themselves, or the emotional maturity to control their emotions, they are expressing their anger and frustrations in the only way they know how which is physically.
Please read my articles "Managing two-year-olds" and "Throw out the timeout" on my website for more information on what is going on with your little ones at this stage and why timeout is not recommended.
If you smack or bite them back you are reinforcing the beaviour you are trying to eliminate. Your boys need to be managed by an attentive adult at all times who can acknowledge their frustrations and anger and teach them empathy. Separating them as well at times will help.
Do not worry too much about playgroup next year. A good teacher should be able to manage them and teach them empathy. This does not mean that you can't do the same at home and it would be beneficial if both you and your husband took some time off to be with them in the next couple of months. My guess is that your nanny has not taught them empathy, which is the ability to put yourself in someone else's shoes and understand their pain. Children as young as two-years-old can be taught empathy.
It is important moving forward that both you and your husband spend one on one time with each twin as well as your older son. This can be done by setting aside ten minutes each day where you give each child undivided attention.
In addition, encourage each twin to develop their own identity by asking to have them placed in separate classes at school. Take each child shopping separately so they can choose their own clothes and things for their room. Let them have their own room or divide the room they share with a curtain so they each have their own space and toys.
Encourage them to develop their own interests and don't send them both to the same activity at the same time. Each boy should have a quiet space in the house they can escape to when their siblings are bothering them. Teach them appropriate ways to express their anger and to remove themselves to their quiet space if they feel as though they can't control themselves.
The more you give them individual attention and help them develop their own identity, the easier they will be to manage. Avoid comparing your boys by looking for each child's individual talent and manage their behaviour at all times in a gentle firm way acknowledging their feelings and showing appropriate ways of behaving.
Good luck. Let me know how you all are doing.