Galvanometer is a device used to detect and measure small currents and also direction of current, it measures current of the order of micro ampere. Due to its high sensitivity, high current may damage it.

Ammeter is used for measuring current and it is connected in series with the component for which we wish to know the current flowing through.

As the ammeter is connected in series (in series resistance is added directly) with the circuit so its resistance should be as minimum as possible so that it can't affect the net resistance leading to neligible effect on current already flowing in the circuit. Resistance of an ideal ammeter is zero.

Shunt is a low resistance connected in parallel with a device to reduce the amount of electric current flowing though it(device).

Note- " A galvanometer can be converted to an ammeter by connecting a shunt parallel to it."

Not in series because in that case it can increase net resistance.

Voltmeter is a device used to measure potential difference between two points in a circuit. It is connected in parallel with the device for which the potential difference betweeen its both ends is desired.

As voltmeter is connected in parallel so to ensure a very low or negligible current through it, its resistance is kept very high, for an ideal voltmeter its resistance is infinite.

"If current through voltmeter is considerable then it would mean that it hampered net current leading to incorrect measurement of potential difference(PD)."

Note- "A galvanometer can be converted to a voltmeter by connecting a high resistance in series with it."