Why does my bank hold a deposit for three days if the transfer looked instant Money & Paperwork

It's waiting to find out whether the money is real. Two separate things are happening and your app is showing you both in the same column, which is why it's confusing.

The number you can see is the ledger balance, which is what has been recorded against your account. The number you can spend is the available balance, which is what the bank is willing to let go of before it's certain the funds have actually settled.

A cheque in particular can come back unpaid days after it was deposited. If the bank released the cash immediately and the cheque bounced, they'd be chasing you for money you'd already spent. The hold is them absorbing that risk on a schedule instead of case by case.

Electronic transfers depend entirely on the system used. Some run in batches a couple of times a day and settle the next business day, some are genuinely instant and irrevocable. Weekends and holidays don't count as business days, which is how a Friday deposit turns into Tuesday.

How long any of this takes varies a lot by country and by bank, and your account terms will spell out the actual availability rules if you want the specific numbers.

The bit people find genuinely surprising: a cheque can be returned unpaid weeks later in some systems, long after the funds became available. Availability is a promise about when you can spend it, not a guarantee that it's yours forever.

Practical version if a hold is going to cause a problem, call and ask. Front line staff often have discretion to release part of it, especially if the payer's bank is one they see all the time. Worst case they say no and you've lost four minutes.

Worth knowing that hold lengths are often shorter for money you receive regularly. Banks build a picture of what's normal for your account, so a payroll deposit from the same source every month usually clears faster than a one-off cheque from someone you've never received money from before.