The question, as stated, cannot be answered sensibly. A millilitre is a measure of volume, with dimensions [L3]. A milligram is a measure of mass, with dimensions [M]. The two measure different things and elementary dimensional analysis teaches that you cannot convert between measures with different dimensions such as these without additional information.
As a simple mental exercise consider a millilitre of air and of water. They will have very different masses.
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Does not convert; milligrams (mg) and grams (g) are measures of weight or mass and mL (milliliters) is a measure of volume.
You can not directly equate mass (mg) and volume (ml) without knowing the density of the substance being measured. For instance 9mg of liquid mercury would occupy a much smaller volume than 9 mg of water.
You have to give 3 times 5 mg = 15 mg therefore you have to give 3 times 2 ml = 6 ml (15 mg = 6 ml of whatever that bottle contains).
1 liter of water weighs 1 kg so 1 ml of water weighs 1 mg so 5 ml of water weighs 5 mg
depends on the substance density