In stanza 3, what does the word "blundered" mean?

A blunder is a mistake, and a careless one. It’s caused by someone not paying enough attention or overlooking important details, and this is exactly what happened to the “Light Brigade” of the poem. Tennyson is basing his poem off of a real, recent event that caused an uproar in England as the result of a poorly made decision all the way in Crimea.
A light brigade is a cavalry unit, lightly dressed and lightly armored. They carry pistols with them, but primarily exist to cut down enemy combatants with sabers that they wear at their hips. Often a light brigade is meant to take smaller forces, to overwhelm infantry on the ground, or to act as a sort of clean up crew at the end of the fight, finishing off units as they break up. A light brigade is not made to oppose large scale artillery, but that’s exactly what they were called to do in the Battle of Balaclava in 1854, at a significant cost of life.
Tennyson’s admiration for the light brigade comes from brigade's courage in the face of the blunder. They knew someone had made a mistake in giving their orders, but it was not up to them to question orders, but to follow them. For Tennyson, this is both respectable and patriotic.

This poem is about the charge of the Light Brigade during the Crimean War in the Battle of Balaclava, which was one of the tragedies of the Crimean War. The Light Brigade was misdirected by the central command; the group was instructed to charge at a well-armed Russian artillery battery rather than the vulnerable one that they were intended to charge. The result was that significant British casualties resulted from the attack, and the promised gains were not made.
It is to this miscommunication in the chain of command that Tennyson is referring when he says that someone had "blunder'd." A blunder is a mistake--often a catastrophic mistake. In this case, the blunder refers to the scrambling of the wires that caused this company to be directed toward the wrong artillery battery. As a result, they had to withdraw quickly and were unable to claim victory.