[*]Get 2 lines and not 3, more lines you get more head ache it is. and is not efficient either for eg. 1 Mbps line can hold more users than 2 512 together. This way its easy to share printers etc too
[*]Keep the first line as your main line (has to be airtel), other use it as for less computer and as back up when primary is down.
[*]What speed is a tricky question, a line with 512Kbps can hold up to 10 computers for normal web usuage, no im not kidding, not everyone downloads in cafe or watches youtube. So i'll suggest you to for got Airtel 512 Unlm (you can always upgrade if u think its slow) second line go for 384 Kbps from different ISP and connect just 2 pcs.
[*]Do the networking so u can easily switch between lines when they r down so ur cafe will never face downtime.
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