I have bc547's! This is what I was hoping for!! Would the relays need their own power source, or would a series resistor from Uno's 5v be okay?
Give the relay modules to someone you don't like, and buy 5volt relay modules. The 3.3volt/150mA supply of an Uno will be struggling to power one 130mA relay module, but can't power two.
The 5volt supply can supply up to about 400mA if the Uno is on USB supply.
You could already be drawing ~150mA with the sensor and Oled.
@LarryD A resistor can fix voltage, but not current.
@johnerrington "Each relay requires "about" 65mA for its control." can't be right.
Most, if not all relay modules I know require 2mA or less control current.
Leo..
For this application, if the relay voltage was 3V and the power supply was 6V and the relay resistance was 100 ohms, a series 100 ohm resistor would let the relay work.
Not ideal but doable.
Hell, a 3v relay might survive 5v 
.
Sir Leo, I can power the relays separately if need be, and it just so happens I don't know a single person I don't like.
I believe the relays take 65mA to power @3V, was my understanding. Where is this 130mA you cite?
So if what I am gathering is correct, the 3V from the uno is "shunted" and can't provide the same, well, wattage as the 5v?
What is the justification for 5v relay over 3?
Regardless, I will try with separate power source and post back... after sleep. Thanks all.
2 (relays) x 65 mA = 130 mA 
UNO's 3.3V is provided by onboard LP2985 150 mA regulator.
Regards
Hi, @audiophileofficial
Can you measure the 3V3 supply when you activate the relay?
I would at this stage disconnect all other hardware and write some simple code to activate the relay.
A separate 3V3 supply seems to be the best solution if the UNO 3V3 is not sufficient.
Thanks.. Tom.. 
Don't believe sales sites. Consult the datasheet. Post#20.
3volt blue sugarcube relays come in two flavours. 120mA and 150mA.
Simple maths.
If a common 5volt relay uses 75mA, then a 3volt relay must use 5/3 * 75mA = 125mA.
Leo..
The relay boards linked in the OP seem to have an optoisolator and driver transistor.
So coil resistance and/or relay current requirements ought to be pretty irrelevant as long as the VCC input to the board has sufficient power capability. IMO something else is wrong. (Did you remember to set the pins to OUTPUT mode?)
You can power it from the 5V. While you could use a series resisitor I'd just put 2 or 3 silicon diodes to drop 1.4 - 2.1V
A CO2 sensor, an Oled screen, and two 3volt relays with resistors that draw the current of four 5volt relays. Sounds a bit much for an Uno. And we don't yet know how the Uno is powered.
Half the current draw.
Leo..
I... uhm... -cough- fudge, I should've posted my code. I'm sorry y'all.
I knew it should've worked, that's why initially I was really only concerned with one relay, nothing else connected. Good to know ahead of time that the project as a whole may cap it out...
Ah, duh. So the 400mA ISN'T available at 3v, it IS indeed "shunted". Funk...
I could run these relays with transistors, with their own 5v/3v supply (with diodes before the relays)... And in that case, can I connect the ground of an uno to the ground of the separate supply, since it won't trigger otherwise?
If I use a couple diodes off the uno's 5v, to the relays, am I still looking at the same problem?
This will be running air and humidity in a mushroom tent, I would like it efficient, yes, but rock solid and stand some test of time.... If anyone input on how to do it more proper, I'm all ears!!
If I gouge out the ridiculously bright LEDs on the relays, would it still function xD and save me some mA? 
You haven't told us anything about LEDs.
Must be big ones if you need relays to switch them on.
Which LEDs, and how do you power them.
All this information should have been in the first post........
Leo..
The LEDs on the relays themselves, that turn on when IN is triggered, I figure they take a good chunk of the mA? I was mostly joking, but I believe I read the Uno uses 60 ish with just it's LEDs and the LEDs on the relays look exactly the same.
'pends on whether the LEDs are in series with the input circuit. In series, removing them will fail the circuit. In parallel, yes, you're right, their current will add to your problems. A bit.
Depends on which relay module.
The 5volt relay modules with opto coupler have the LED in series with the opto coupler,
so they don't take any extra current at all.
Your module seems the same, looking at the picture in post18.
1.8volt LED>1.2volt opto>100ohm resistor = 2mA signal current.
Leo..
It was a thought, and even if in parallel, I don't see how it would be easy to figure if it would affect the relays normal operation.
I think in reality I just want to stab the relays because they aren't exactly what I need. If I knew ANYone with interest in DC hobby stuff I'd would give the relays to them but there isn't even an electronics shop in my city anymore... maybe my old college prof xD
Somehow you are going to give energy to the circuit, I imagine with a 5V source.
You can add a step-down to lower the voltage to 3.3.
You can also try with 2 diodes type 1N4001 in series taking the 5V of the arduino.