Actually you can achieve something like round robin with one simple workflow and custom field.
Let's assume, that you have bunch of leads to import and you want to assign them between 3 user equally.
First of all, you have to create integer field on user form. Next, input values 0 and 1 to this field for all users, as you want to, like that:
User A custom integer = 0
User B custom integer = 0
User C custom integer = 1
Now it's time to build workflow, it can run on create.
If User A custom integer = 1 assign to him, set User A custom integer = 0, set User B custom integer = 1, set User C custom integer = 0
else if User B custom integer = 1 assign to him, set User A custom integer = 0, set User B custom integer = 0, set User C custom integer = 1
else if User C custom integer = 1 assign to him, set User A custom integer = 1, set User B custom integer = 0, set User C custom integer = 0
If you set values as I described, you should get as results Lead assigned to User C, and next one in line is User A.
When you and another lead, then it will be assigned to User A, and next in line is User B, and so on.
You can replace custom integer with bool field or any other. Key is to construct workflow with stable values.
Crazy as it sounds, but it should be working :)