Actually, modify 2.525.37 and re-release since the kernel flavour name was wrong.

debian/changelog

@@ -1,7 +1,7 @@
 livecd-rootfs (2.525.37) bionic; urgency=medium
 
-  * Use the linux-raspi2-5.3 kernel flavour for the raspi3 images.  This kernel
-    is required for proper pi4 support. (LP: #1859478)
+  * Use the raspi2-hwe-18.04-edge kernel flavour for the raspi3 images. This
+    kernel is required for proper pi4 support. (LP: #1859478)
 
  -- Łukasz 'sil2100' Zemczak <lukasz.zemczak@ubuntu.com>  Mon, 13 Jan 2020 15:33:20 +0100
 

live-build/auto/config

@@ -882,7 +882,7 @@ case $ARCH in
 				;;
 			raspi3)
 				COMPONENTS='main restricted universe multiverse'
-				KERNEL_FLAVOURS=raspi2-5.3
+				KERNEL_FLAVOURS=raspi2-hwe-18.04-edge
 				add_package install linux-firmware-raspi2 u-boot-rpi flash-kernel u-boot-tools wpasupplicant
 				BINARY_REMOVE_LINUX=false
 				;;