Looking for help how to migrate 2disk RAID1 to 3 disk RAID6

I have a system with two drives like this:
/ RAID1 on sda3 and sdb3
/boot RAID1 on sda1 and sdb1

I would like to migrate to
/ RAID6 on sdi3, sdj3 and sdh3
/boot RAID1 on sdi1, sdj1 and sdh1

The /boot already done.
How would I do the migration to RAID6?

The new drives i,j and h are larger then a and b, after the migration I
will reshape the raid to use all the space on the new raid, unless it is
possible to do it in one step.

The reason I want RAID6 and not RAID1 with 3 drives is that later I may
add more drives to the array.

Currently I don't trust any RAID that has only one parity drive, it has
to survive a failure of any two drives. So RAID6 or triple RAID1 are a
minimum.

Thanks in advance,
K