What thickness cable should I use to wire these outside lights?
Current
3 x 30W = 90W
90W / 240V = 0.375 A
So 20A cable is overkill.
A typical lighting circuit uses 5A cable - typically 1 mm² (older installations) or 1.5 mm² (current standards). You could use thinner cable rated for 1A but you might then need a fused connection unit with a 1A fuse to protect the 1A cable from setting fire to your house in the event of a fault in the light fitting.
Outdoor use
What is important for outside use is insulation durability. It has to stand up to UV from sunshine and extremes of hot and cold plus mechanical strength. You need cable rated for outdoor use.
Totally random example
Another random example

300/500V for environments like pond pumps and external lighting exposed to sun rays with standard risk of mechanical damage.

HAR
Rubber insulated
-20 to +60°C
BSEN 50525-2-21


Shedding water
Cable needs to be routed so that it exits the wall and any enclosure in a downward direction so that rainwater does not run into the wall nor into any enclosure. Enclosures need to be waterproof (e.g. IP66) and fitted with cable-glands to prevent water ingress.
Walls
Running cable through masonry walls should be protected in conduit. At the inside there should be something to indicate the presence of any electrical cables channelled into the plaster - e.g. a back-box with blanking plate.
Ideally you'd use T&amp;E cable through conduit through the wall and flexible cable to a lamp. Which obviously requires an external waterproof junction box.