Draw a model to help you understand the question better. Since the price of a packet of biscuit is three times the price of a packet of snack, a packet of snack will be 1 unit and a packet of biscuit is 3 units.

1 water + 1 snack + 1 biscuit = $5.40
1 water + 1 unit + 3 unit = $ 5.40

And since water is $3 less than biscuit, you can say that the price of a bottle of water + $3 will be equal to the price of a biscuit. So you can add $3 to both side of the equation.
(1 water +$3.00) + 1 unit + 3 unit = $5.40 + $3.00
1 biscuit + 1 unit + 3 unit = $8.40
3 unit + 1 unit + 3 unit = $8.40
7 unit = $8.40
1 unit = $1.20

price of a bottle of water = 1 biscuit - $3.00
price of a bottle of water = 3 unit - $3.00
price of a bottle of water = (3 x $1.20) - $3.00
price of a bottle of water = $3.60- $3.00
price of a bottle of water = $0.60

If you understand the basics of algebra, you can use the following methods below
Let the price of a packet of biscuit be X

1 water + 1 snack + 1 biscuit = $5.40
(3X - $3.00) + X + 3X = $5.40
3X + X + 3X = $5.40+$3.00
7X = $8.40
X=$1.20

Price of water bottle = 3X -$3.00
Price of water bottle = $3.60 - $3.00
Price of water bottle= $0.60