Session Access in BAL(3-tier) Layer

You can do couple of things

Simplest way is
Add a reference of system.web to BAL,
then use the following syntax

HttpContext.Current.Session["MySession"]

Secondly Proper way is

i)Create a class library which has a reference to System.Web. create Class Say ClsStateManagement in this project.
Add a property to the class

public string UserId
   {
       get
       {
           return (string)HttpContext.Current.Session["UserId"];
       }
       set
       {
           HttpContext.Current.Session["UserId"]=value;
       }
   }

ii) Add reference of this project to both you UI and BAL

Now in you ASPX.CS (in UI)

ClsStateManagement c=new ClsStateManagement()
c.UserId="MyUserId";

in BAL

ClsStateManagement c=new ClsStateManagement()
string myUserId=c.UserId;

Mark this as corerct answer if it helped.