Is a three-at-the-back system inherently defensive?

Three centre-backs can mean a back five, or it can free the wing-backs to attack.

Does the shape itself decide how defensive a team is, or is it all about instructions?

Spot on with the wing-backs point. It entirely comes down to whether those wide players are told to hold the line or bomb forward. If you look at accumulators, backing a team in a 3-5-2 can be a trap if the manager has set them up to sit deep and absorb pressure. But when the wide centre-backs are progressive and can carry the ball into midfield, it completely changes the dynamic and creates proper overload situations. Anyone tracking weekend stats on teams shifting between a back four and back three recently? Curious if the conversion rate on both-teams-to-score changes much when they make that switch.

It is 100 percent about the player instructions and the game state, not just the numbers on the tactics board. When a team chasing a late goal shifts from a flat back four into a back three with adventurous wing-backs, the whole momentum of the pitch flips. Those wide center-backs stepping up into midfield create that extra body to sustain attacks deep into stoppage time. Does anyone notice those teams converting more late pressure into goals when they make that late tactical shift, or do they just leave themselves too exposed on the counter?

Spot on about the instructions dictating everything. From an away end, you can spot the difference straight away just by watching how high the wide centre-backs step up when the team has possession. If they stay timid and flat, the whole away section ends up staring at a boring back five that invites pressure. When a manager actually gives those outside centre-backs license to overlap or carry the ball into space, the atmosphere in the stands completely shifts because the team looks like they're actually trying to win rather than just hold on. Do you reckon managers are getting braver with those wide center-back roles away from home, or are most still setting up too cautiously on the road?