Can I connect two 4 ohm speakers and a 8 ohm speaker?

I depends on "how" you are connecting them. If you connected the 4 ohm speakers in series (positive terminal of one to the negative terminal of the other). This will leave a negative terminal of one unconnected and a positive terminal of the other unconnected. then connect them in parallel (the unconnected positive terminal of the 4 ohm speakers to the positive terminal of the 8 ohm speaker and the unconnected negative terminal of the other 4 ohm speaker to the negative terminal of the 8 ohm speaker. This will produce an overall 4 ohm load for 1 channel (assuming you are connecting to a stereo)

Or if you are connecting to a stereo with two 8 ohm outputs. Connect the 8 ohm to one channel then connect the above mentioned series connected 4 ohm speakers to the other channel for a total load of 8 ohms on both channels.

Parallel connections will equal less than the individual speaker load. The formula for this is: R X R/R + R where R is ohms for each speaker ie. Two 8 ohm speakers in parallel would be 8 x 8 (64)/ 8 + 8 (16) which is 4 ohms.

Series connections you just add them together ie. Two 8 ohm speakers in series would be 8 + 8 or 16 ohms.

FYI. If you measure the speakers with an ohmmeter it will be less than what it says on the speakers.

Speakers are measured in "impedance" which takes into account "reactance". The resistance of the speaker coil will change slightly with different frequencies applied so an 8 ohm speaker may read as 5-7 ohms (more or less) with an ohmmeter. A 4 ohm may read as low as 2.5 ohms on the same meter.

Some output Transformers can handle half the ohms but it is always safer, as a general rule, to go "up" in speaker impedance without any damage. It's not wise to vary any with a system with solid state (transistor or IC) output coupling.

Hope I didn't get too technical. There is lots of good info on the internet about speaker configurations.