What is the maximum transaction throughput of the Bitcoin network?
The smallest transaction that would commonly occur in the wild is a P2PKH transaction with one input and two outputs (send amount and change output). A P2PKH input has 148 bytes, a P2PKH output has 34 bytes and the transaction overhead is 10 bytes, 148 bytes + 2*34 bytes + 10 bytes = 226 bytes, in the case when no change output is required, this could actually be 192 bytes.

The blocksize limit is a megabyte, not a mebibyte, so it's 1,000,000 bytes.

If all transactions were only P2PKH with one input and two outputs, the throughput would be 1,000,000 / 226 / 600 = 7.37 tps. However, the mix is very different with a lot more transactions on the network making use of multisignature inputs or sending to boatloads of different addresses at the same time, and the current average transaction size is around 520 bytes, making the 3 tps figure much more accurate.

I don't know why the authors would have only rounded the upper bound.