I know someone who adopted three, they did not all come at the same time but very close to each other.
It is very noble that you want to keep a family together BUT please do take advice from everyone about the specific children you adopt.
As Alljamissweet says "... keep an open mind, it might be a sibling group, it might not. It just has to be right for you."
Adoption is all about creating a new family, not just about caring for a group of children who are already a family, I know you know that but I am just pointing it out. Your children will become your kids and you will create something new with them. If they should be able to stay with their siblings then of course, they should. But it isn't always in their best interests. Sometimes they will have needs which cannot be met by a standard family, so they may end up being separated from siblings and adopted by couple or by a single adopter, because that is best for them. Some will not be able to have a normal sibling relationship with each other in the home but if adopted separately will be able to see each other and develop and, one hopes, thrive.
Some sibling groups that are taken into care can contain five traumatized kids, no normal couple could cope with that. It is not like a family where the parents die and a relative takes them all on to keep them together, and they are a loving and functional family unit. Of course it might be, but living in a chaotic and dysfunctional family where maybe one child has been favoured greatly or one treated badly... it's all so varied.
Can I ask what your experience with kids is?
If you definitely want three, then get all the experience you can with groups of children, challenging ones if you can. Not just well behaved nieces or nephews (my nephews are not very well behaved!!) or very behaved Brownie packs. See if there are any groups or clubs for children with any behaviour issues etc and see if you can volunteer there.
It still won't be the full 'experience' as they are unlikely to just let you look after three kids on your own, but it could help you gauge how well you can cope and when questioned by social workers about your abilities you can talk about what you have done if it is appropriate.
There are some documentaries about people who adopted multiple children. I will try and link to some if I can.
Good luck, and KEEP ON CARING, it's great you want to do this. 