How three vendors happen
Sequentially. Someone good at social found you first. Then a search specialist made a compelling case. Then the website needed help and a third party took it on. Each is competent within their box.
The trouble is that nobody owns the space between the boxes, and in paid media most of the value is in that space.
The cost of the seams
Budget cannot move between channels, because moving it means renegotiating with two vendors. Nobody sees the whole customer journey, so the search vendor takes credit for demand the streaming ads created. Creative is produced three times over. And when results are flat, every vendor can point at another one.
You also pay three minimums. Three floors on three retainers is often more than a single provider charges to cover everything.
What one owner changes
With one team running every channel, three things become possible that were not before. Budget can shift mid-month toward whatever is converting. Frequency can be managed across channels rather than duplicated within each. And a single report can attribute an outcome once instead of three times.
None of this requires better people. It requires that the same people see everything.
Doing it without a disruption
Consolidate in one move, not three. Pick the provider, give them read access to everything, have them rebuild campaigns in parallel while the current setup keeps running, then cut over at a month boundary. Keep the old accounts intact for 60 days so historical data stays available.
The transition is a week of work. The compounding benefit is every month after.