Unfortunately, there's a bit of a Catch-22 here. I prefer simply to say that the bread is the body of Christ and the wine is the blood of Christ and wish we could just simply leave it at that. Since others have adopted positions that say more and/or less than Scripture says, it becomes necessary to explain what you are saying when you say that the bread is the body and the wine is the blood.
"In" indicates that the body and blood of Christ are in the bread and wine of the sacrament. Christ did not say that this is bread is no longer bread. Christ, holding the bread, said, "This is my body." "With" indicates that we receive body, blood, bread, and wine. When Paul speaks of the consecrated elements he goes back and forth between speaking of the bread and speaking of the body of Christ. "Under" indicates that the body and blood are there but they are there in a hidden way. We cannot detect them through scientific tests or visibly see them. I think may understand "under" to mean that if you look underneath the bread you're going to find Jesus there. However, that's not what the language intends to communicate in its original context.
"In, with, and under" is often understood to be teaching consubstantiation, local co-existence, or impanation. However, our confessions deny all of these things. The eating of Christ's body is not a physical eating but mystical and sacramental. This does not mean that it's some mere figurative eating. But we recognize that Christ's body is not locally and physically present. Christ's body and blood are supernaturally, mysteriously, and incomprehensibly present. There is a real sacramental union. The bread and the body and the wine and the blood exist together. We confess this to be the case because this is what the Scriptures say and refuse to go beyond this. Unlike consubstantiation and transubstantiation we hold to no theory about the coexistence of two substances.
As Luther says:
Therefore, it is entirely correct to say, if one points to the bread, “This is Christ’s body,” and whoever sees the bread sees Christ’s body, as John says that he saw the Holy Spirit when he saw the dove, as we have heard. Thus also it is correct to say, “He who takes hold of this bread, takes hold of Christ’s body; and he who eats this bread, eats Christ’s body; he who crushes this bread with teeth or tongue, crushes with teeth or tongue the body of Christ.” And yet it remains absolutely true that no one sees or grasps or eats or chews Christ’s body in the way he visibly sees and chews any other flesh. What one does to the bread is rightly and properly attributed to the body of Christ by virtue of the sacramental union.