I would consider the deal that Antonio and Shylock make, in which Shylock lends Antonio money under the condition that Antonio surrender a pound of flesh should he not pay it back in three months time, the main plot driving the action of the play, not a subplot. The three subplots are as follows:
The casket plot: in this subplot, Portia cannot chose her suitor but must marry whoever picks the right casket, according to instructions left in her father will.
Lorenzo and Jessica's elopement: Shylock's daughter Jessica elopes with Antonio and Bassanio's friend Lorenzo, enraging Shylock.
The ring plot: both Portia and Nerissa, in disguise, demand the rings that Bassanio and Gratiano promised never to part with.
The three subplots are connected because they all involve love. Portia, for example, complains to Nerissa that she cannot chose her own husband. Although she ends up with the man she wants, this subplot critiques patriarchal control over women: Portia amply demonstrates when she successfully defends Antonio that she has the intelligence and judgment to choose her own mate. Jessica also resists patriarchal control when she defies her father to run off with the man she loves. Portia and Nerissa demonstrate their love, mercy, and forgiveness towards their lovers when they return the rings the lovers gave away.
All three subplots are also strongly connected with Belmont, Portia's home of enchantment and romance outside Venice. The casket plot takes place at Belmont, Jessica and Lorenzo end up at Belmont, and Portia and Nerissa return the rings at Belmont. This place of love is a contrast to Venice's mercantilism and commerce.