Blend sugar, cornstarch, and salt in 2-quart saucepan. Add boiling water; stir and cook on medium-high heat until mixture boils and is thick. Put lid on pan; turn heat to very low and let mixture cook 10 minutes, stirring once. Stir about 1/2 cup of the hot mixture into the egg yolks, then return mixture to pan and continue cooking for a minute or two, stirring constantly. Remove from heat, and stir in lemon juice, rind, and butter, and stir until butter is melted. Pour into pie shell. Make meringue: Beat 3 egg whites with cream of tartar until foamy and about double in volume (use small bowl of electric mixer). Add sugar gradually, about 1 tablespoon at a time, beating all the while with mixer set at highest speed until sugar completely dissolves and meringue stands in firm peaks. Spread meringue on pie filling, being sure to seal it to the shell. Swirl meringue into a pretty design with peaks and valleys. Return to oven and bake either at 375 degrees F. for 15 to 20 minutes or at 425 degrees F. for 3 to 5 minutes. Let peaks of meringue brown to a delicate golden color. Remove pie from oven and cool on a rack for at least 4 hours before serving. (Note: We prefer the lower-temperature, longer-time method; there is less chance of soft, uncooked egg white, although greater shrinkage usually results with this method.)
