If they're working, then the three chips SIMMs should be fine.
There are occasional reports of three-chip (or two chip) SIMMs not working in old Macs, and I have always wondered about that.
A SIMM is very simple, and a 30 pin SIMM especially so. There are twelve address pins, eight data pins, and a handful of control pins (R/W, RAS, CAS).
The SIMM circuit board connects these pins to the corresponding pins on the memory chips. What could be simpler?
The address and control pins fan out and connect to all of the memory chips. Every memory chip has all the address pins (may be fewer than 12 on lower capacity SIMMs) and the three control pins.
On an eight (or nine) chip SIMM there is one unique data pin for each memory chip. In other words, each memory chip is responsible for storing just one bit out of every byte in memory.
Two chip SIMMs exist because some memory chips store 4 bits at a time, instead of just 1 bit at a time.
On a two chip SIMM four of the data pins go to one memory chip. The other four go to the other memory chip.
There's no logical reason why a 2-chip SIMM shouldn't work anywhere an eight chip SIMM works. "Logical" in the digital sense. There may be an analog reason why a two-chip SIMM won't work.
On a two-chip SIMM, two chips are connected to the address and control pins. On an eight-chip SIMM, eight chips are connected to the address and control pins.
Each chip consumes a little bit of electrical current at the I/O (address/control/data pins) pins when receiving signals. This action is referred to as "the pins sink current". It's a tiny amount, because the ideal is for the I/O pins to just sense the voltage level without consuming any of the current. So designers strive for the ideal, but never quite achieve it.
So a two-chip SIMM sinks about 1/4 the current from the address and control lines as an eight-chip SIMM does.
It is possible that this difference in sunk current causes ringing or other signal problems in some Macintoshes, such that they will not work with some 2-chip SIMMs. The Mac memory control circuits are designed to provide enough current on the address and control lines to drive eight chips per SIMM. If the SIMM doesn't consume enough of that current, it might cause voltage swings.
An interesting experiment, if one ever finds a situation in which 2-chip SIMMs don't work, would be to replace 2-chip SIMMs with 8-chip SIMMs, one at a time, until/if the board starts working again.
If my hypothesis is correct, there should be a point at which replacing 2-chip SIMMs with 8-chip SIMMs results in a high enough sunk current that the board would start working again, even though some 2-chip SIMMs are still installed.