I can find the quote "in, with, and under" on several Lutheran Church sites, but I haven't seen it in a writing of Luther himself. Does the phrase appear in Luther's works?
Instituted by Jesus Christ himself, it is the true body and blood of our Lord Jesus Christ, under the bread and wine, given to us Christians to eat and to drink
In his Larger Catechism Luther writes under the heading "The Sacrament of the Altar" (this section is ommitted by some printers, who for some reason only include his teaching on the Ten Commandments):
"das Sacrament des Altars ... ist der ware leib und blut des HERRN Christi ynn und unter dem brod und wein..." (pardon the archaic German spelling)
That is: "The Sacrament of the Altar ... is the true body and blood of the LORD Christ in and under the bread and wine". (NB The word 'unter' can also be translated 'among' and where the word 'bei' is rendered 'with' it can also be translated as 'alongside' or 'at')