Found the following hamletworks.org entries referring to line 3866

View Commentary Note for 3864, etc.
3864 To tell him his commandment is fulfild,5.2.370
3865 That Rosencraus and Guyldensterne are dead, 
3866 Where should we haue our thankes?5.2.372

View Immaterial Textual Note for 3864, etc.
3864 To tell him his commandment is fulfild,5.2.370
3865 That Rosencraus and Guyldensterne are dead, 
3866 Where should we haue our thankes?5.2.372

View Material Textual Note for 3864, etc.
3864 To tell him his commandment is fulfild,5.2.370
3865 That Rosencraus and Guyldensterne are dead, 
3866 Where should we haue our thankes?5.2.372