The larger image is the proper way to go.
What is important is the single image has only one round trip http request
where there will be three for three images.
In a test the 3 images took 2.68x or 268% more load time.
In this page load waterfall I have 3 images about 3K each.

This is one 12K image.

Orange is the time for Browser to connect to server.
Green is the wait time for server to put image in output buffer.
Blue is the time for the server to transmit to the Browser.
The 3K images have only a very small thin blue line which is 3 milliseconds.
Three 3K image times
430 mS total load time.
connect 65 mS
wait 72 mS
transmit 3 mS
Transfer 3515 bytes @ 878,750 Bytes/sec.
connect 64 mS
wait 71 mS
transmit 3 mS
Transfer 3515 bytes @ 878,750 Bytes/sec.
connect 76 mS
wait 73 mS
transmit 3 mS
Transfer 3123 bytes @ 780,750 Bytes/sec.
The 12K image
160 mS total load time.
connect 69 mS
wait 74 mS
receive 17 mS
Transfer 11982 bytes @ 665,667 Bytes/sec.
430 / 160 = 2.68 * 100 = 268%